IRS Notice of Deficiency (90-Day Letter): What It Is and What to Do

Written by Sam Brotman, J.D., LL.M. (Taxation), MBA — Managing Attorney, Brotman Law  ·  Updated May 2026

What a Notice of Deficiency Is

The short version is that a notice of deficiency is the document that separates an IRS examination from a legally enforceable assessment. Before the IRS can make you pay what it says you owe, it has to give you this notice and the chance to dispute it in court. That's the deal Congress set up under the deficiency procedures in IRC §§ 6211–6215.

The notice itself will include the tax years at issue, the proposed deficiency amount broken down by year, and the specific adjustments the IRS made. It comes on official letterhead and will reference the statutory authority. What it won't necessarily do is explain the IRS's reasoning in plain English. That's what the revenue agent's report — the Form 4549 — is for, and that should have preceded the notice.

The informal names matter for search purposes but mean the same thing: "90-day letter," "stat notice," and "notice of deficiency" are all the same document. If someone says they received a "statutory notice of deficiency," they're referring to the same thing.

What Triggers a Notice of Deficiency

Audit Conclusion Without Agreement

The most common path to a notice of deficiency is an IRS audit that ended without your agreement. When an audit closes and you haven't signed Form 870 (waiver of restrictions on assessment) or Form 4549 (income tax examination changes), the IRS issues the notice of deficiency to formally begin the deficiency process.

Before the notice issues, the IRS typically sends a 30-day letter — the report of proposed adjustments with an invitation to request an IRS Appeals conference. If you did not respond to the 30-day letter or went through Appeals and still didn't agree, the next step is the notice of deficiency. At that point, the administrative process is done and the statutory process begins.

CP2000 Escalation (Automated Underreporter)

The IRS's automated underreporter (AUR) program compares your return against third-party reporting — W-2s, 1099s, K-1s. When there's a discrepancy, the IRS issues a CP2000 notice proposing additional tax. If you don't respond or the IRS finds your response unsatisfactory, it escalates to a notice of deficiency. The same 90-day rule applies.

An important distinction: a CP2000 is a proposal, not an assessment. A notice of deficiency is the step that triggers the statutory deadline. If you are still in the CP2000 stage, you have more flexibility — but if the case moves to a notice of deficiency, that flexibility is gone and the clock is running.

The IRC § 6201 Requirement

The reason the notice exists at all is IRC § 6201's requirement that the IRS obtain authority before it assesses income tax deficiencies. The deficiency procedure — notice of deficiency plus the 90-day waiting period — is that authority. The IRS can't skip it for income taxes (there are specific exceptions for jeopardy assessments, but those are narrow).

The 90-Day Deadline

A few things that catch people:

• The clock runs from the date on the notice, not the date you received it. The IRS mails notices by certified mail, and the date on the notice is what controls. If the mail was slow, that's your problem, not the IRS's. Open your mail promptly and call a tax attorney the same day you receive one of these.
• The 90-day period includes weekends and holidays — but if the last day falls on a Saturday, Sunday, or legal holiday, the deadline extends to the next business day.
• The filing date is the date the petition is received by the Tax Court (or postmarked if mailed by certified or registered mail). Not the date you dropped it in a mailbox. For something this consequential, use overnight mail with delivery confirmation or file electronically through the Tax Court's DAWSON system.
• One notice, one 90-day period. The IRS can only issue one notice of deficiency per tax year. If it issues a defective one, the clock still runs.

The IRS cannot assess or collect the proposed deficiency while the 90-day period is open — and it cannot act if you've filed a timely petition until the Tax Court case is resolved. That's the protection the statute gives you.

Your Four Options Within 90 Days

Once you have a notice of deficiency in hand, you have four realistic paths. Here's what each one means.

1. File a Tax Court Petition

Filing a petition in U.S. Tax Court stops the IRS from assessing the deficiency and opens a formal legal proceeding. You don't pay first. The case gets docketed, IRS Counsel gets involved, and the matter typically goes back through IRS Appeals before it ever comes close to trial. This is the right move in most contested cases. More on this below.

2. Agree and Sign Form 870

Form 870 is a waiver of restrictions on assessment — signing it means you're agreeing to the IRS's proposed deficiency. Once signed, the IRS assesses the tax and interest starts running. You pay, and the matter is closed unless you later decide to file a refund claim. This is the right move when you have reviewed the IRS's adjustments, they're correct, and you'd rather close the case than incur additional professional fees fighting a correct assessment.

3. Pay in Full and File a Refund Claim

You can pay the full assessed amount (or what you think will be assessed), then file a Form 1040-X or a written refund claim with the IRS. If the IRS denies the claim or ignores it for six months, you can sue in U.S. District Court or the Court of Federal Claims. This approach preserves your ability to litigate but requires you to have the funds to pay first. Not the most common approach to a notice of deficiency, but available.

4. Do Nothing

If you do nothing, the 90-day window closes, the IRS assesses the tax, and collection begins. Your Tax Court rights are permanently gone for that tax year. You've given up the prepayment forum and the protection against assessment while the case is pending. This is not a strategy — it's a forfeiture.

Tax Court vs. Paying and Suing for a Refund

For most taxpayers in a notice of deficiency situation, Tax Court is the right starting point — specifically because you don't have to pay first. Paying a contested deficiency before you've had a chance to challenge it can be difficult or impossible depending on your financial situation, and it shifts the procedural advantage to the IRS.

District Court does offer jury trials, which matters in some cases — particularly where credibility is at issue and you believe a jury would be more sympathetic than a Tax Court judge. But that calculus rarely applies at the notice of deficiency stage. The whole question here hinges on whether the IRS's proposed adjustments are correct, and that is a legal and factual analysis that Tax Court handles well.

Filing a Tax Court Petition

The basics:

• Where to file: U.S. Tax Court, 400 Second Street NW, Washington, D.C. 20217. Or electronically through the Tax Court's DAWSON system at ustaxcourt.gov. Electronic filing is available to represented taxpayers and pro se petitioners alike.
• The filing fee: $60 at the time of this writing. Pay by check or money order made out to "Clerk, United States Tax Court." Low-income taxpayers may apply for a waiver.
• Regular Case vs. Small Tax Case (S Case): If the total amount in dispute for each tax year is $50,000 or less, you can elect the Small Tax Case procedure by checking the box on the petition. S cases are less formal, move faster, and don't require the same level of legal preparation — but S case decisions cannot be appealed to the Circuit Courts. If the amount is close to $50,000 or the legal issues are complex, think carefully before electing S case status.
• What the petition must include: Your name and address, the notice of deficiency date and the IRS office that issued it, the tax years at issue, the specific errors you believe the IRS made, and the facts supporting your position. It doesn't need to be long, but it does need to identify the issues.

You can file a Tax Court petition without an attorney — the Tax Court is designed to be accessible to pro se petitioners. That said, if the amounts are significant or the legal issues are not straightforward, having counsel makes a real difference, particularly during the IRS Appeals phase that follows docketing.

What Happens After You File

The sequence after filing:

1. The case is docketed. The Tax Court assigns a docket number and sends both parties a Standing Pretrial Order with deadlines.
2. IRS Counsel is assigned. An attorney from the IRS Office of Chief Counsel enters the case. From this point, formal communications go through counsel, not the revenue agent who issued the notice.
3. IRS Appeals gets involved — a second time. Even if you went through Appeals before the notice issued, filing a Tax Court petition opens a second round. IRS Counsel will typically refer the case back to Appeals before doing anything else. This is sometimes called the "docketed Appeals" process, and it's where the vast majority of Tax Court cases actually resolve.
4. Settlement discussions. Through the docketed Appeals process, both sides exchange documents, discuss the issues, and negotiate. Most cases settle here without a trial. A settlement is formalized through a stipulated decision document.
5. If no settlement, trial. Cases that don't settle go to trial before a Tax Court judge. Trials can be scheduled at locations around the country, not just D.C. The judge issues an opinion, which can be appealed to the applicable Circuit Court (unless it was an S case).

The practical upshot is that filing a Tax Court petition is not the same thing as going to trial. It's the beginning of a process that almost always ends in settlement. The petition preserves your rights and puts you in a position to negotiate from a standing posture, rather than from one where the tax has already been assessed and collection is underway.

What Happens If You Miss the 90 Days

Once the 90-day window closes without a petition, the IRS sends a Notice and Demand for Payment. From there, the collection timeline begins:

• The IRS can file a Notice of Federal Tax Lien.
• The IRS can issue levies on wages, bank accounts, and receivables.
• Collection Due Process rights attach — you can request a CDP hearing before a levy issues, but that hearing cannot restore Tax Court jurisdiction over the underlying tax liability.

Your litigation options after missing the deadline are limited to the post-payment refund path: pay the full assessed amount, file a formal refund claim with the IRS, wait for a denial (or six months), and then file suit in U.S. District Court or the Court of Federal Claims. That process is more expensive, requires access to funds, and puts the procedural burden on you in ways that Tax Court does not. It's not impossible, but it is harder.

In practice, most taxpayers who miss the 90-day window are not doing so strategically. They didn't understand the deadline, the mail went to the wrong address, or they were waiting to hear back from someone. The IRS mails notices of deficiency by certified mail to the taxpayer's last known address — if you've moved and haven't updated your address with the IRS, the clock runs from the date on the notice regardless of whether you received it.

If you believe the IRS mailed the notice to the wrong address, that issue is litigable — but it's expensive and uncertain. Keeping your address current with the IRS via Form 8822 (Change of Address) is simpler.

For context on the underlying audit that may have led to the notice, see our guide to IRS audit defense, and specifically the IRS audit statute of limitations, which governs how far back the IRS can examine your returns in the first place.

The California FTB Equivalent

The FTB's NPA functions similarly to the federal notice of deficiency in that it starts a clock on your right to contest the assessment administratively. The differences:

• The protest period is 60 days, not 90. Shorter window, same absolute nature.
• The protest goes to the FTB's protest unit, not a separate court. There is no California equivalent of Tax Court for initial protests — you go back to the agency first.
• After the FTB protest: If you disagree with the FTB's decision after the protest, you can appeal to the Office of Tax Appeals (OTA), California's independent administrative tax tribunal. From OTA, you can seek judicial review in California Superior Court.
• Missing the 60-day protest deadline means the NPA becomes a final assessment and the FTB can begin collection — including bank levies, wage garnishments, and liens on California property.

If you have both a federal notice of deficiency and an FTB NPA for the same tax years, you are dealing with two separate deadlines and two separate administrative tracks. The IRS issue doesn't resolve the FTB issue, and vice versa. Coordinating the defense matters — the positions you take at the federal level can affect the state proceeding.

If you've received a notice about an IRS Information Document Request during an audit, that's typically an earlier step in the same chain that can eventually produce a notice of deficiency if the examination concludes without agreement.