A Notice of Deficiency — often called the “90-day letter” or a Statutory Notice of Deficiency under IRC § 6212 — is one of the most important pieces of mail the IRS will ever send you. It is the IRS’s formal determination that you owe additional tax, and it is the only ticket you have to challenge that tax in U.S. Tax Court before paying a dime.
Miss the 90 days, and your only path to dispute the tax becomes paying it first and suing for a refund. Respond on time, and you keep every option on the table.
What a Notice of Deficiency actually is
A Notice of Deficiency is issued at the close of an examination when the taxpayer and the IRS have not reached agreement. It states the proposed deficiency, the tax year(s) at issue, and any penalties the IRS intends to assess. Critically, it triggers the 90-day window (150 days if addressed outside the United States) to file a petition with the U.S. Tax Court.
It is not a bill. It is a legal predicate to assessment. The IRS cannot assess the proposed tax until the 90 days expire without a petition, or until the Tax Court enters a decision.
How to recognize one
- Sent by certified or registered mail (this is required by statute).
- Includes IRS Letter 3219 or CP3219A/CP3219N.
- Contains a Form 5278 or 4549-A with the proposed adjustments.
- Clearly states a deadline 90 days from the date on the notice.
Your three options once you receive it
1. File a Tax Court petition within 90 days
This is almost always the best option if you disagree with the deficiency. Filing a petition stops assessment, preserves your right to a judicial forum, and routes the case back through IRS Appeals — where most cases settle. The filing fee is $60 and you do not have to pay the disputed tax first.
2. Sign and agree
If you agree with the proposed deficiency, you can sign Form 4089 (or the waiver embedded in the notice) and the IRS will assess and bill you. Interest continues to accrue from the original due date of the return.
3. Do nothing — the worst option
If you let the 90 days lapse without petitioning, the IRS assesses the tax. Your only remaining path is to pay the deficiency, file a claim for refund, wait six months, and then sue in U.S. District Court or the Court of Federal Claims. That is slower, costlier, and much harder to win.
The 90 days are jurisdictional
This is the part most taxpayers miss. The 90-day deadline is statutory and the Tax Court has no power to extend it. A petition filed on day 91 will be dismissed for lack of jurisdiction — regardless of how meritorious the underlying case may be. The clock starts on the date printed on the notice, not the day you received it.
Filing the petition
You can file a Tax Court petition electronically through DAWSON, the Tax Court’s online system, or by mail. Use the “timely mailing is timely filing” rule under IRC § 7502 if you mail it — keep the certified-mail receipt as proof. Small case (S) status is available for disputes of $50,000 or less per year and is faster, less formal, but has no right of appeal.
What happens after you petition
The IRS files an answer, and the case is typically routed back to IRS Appeals for settlement consideration. The vast majority of Tax Court cases settle before trial. If the case does go to trial, it is heard by a Tax Court judge — no jury — and the judge issues a written decision.
The bottom line
A Notice of Deficiency is the IRS’s last formal step before assessment, and it hands you a powerful tool: 90 days to put the case in front of a Tax Court judge. Calendar the deadline the minute the certified mail arrives. Then talk to a tax attorney about whether to petition — almost always the answer is yes.