Notice of Deficiency (90-Day Letter)
Definition
A Notice of Deficiency is a formal IRS determination that additional tax is owed, triggering a 90-day window during which the taxpayer may petition the U.S. Tax Court without first paying the disputed amount.
Why This Matters for Tax Relief
The Notice of Deficiency — commonly called the '90-Day Letter' — is the gateway to Tax Court. Receiving this notice triggers the most time-sensitive deadline in tax law. The taxpayer has exactly 90 days (150 days if addressed to someone outside the United States) to file a petition with the U.S. Tax Court. This window is jurisdictional — missing it eliminates the right to Tax Court review entirely. After the 90-day window closes without a petition, the IRS will formally assess the deficiency, making it immediately collectible. At that point, the taxpayer's only recourse is to pay the full amount and file a refund claim in U.S. District Court or the Court of Federal Claims. The significance of the 90-Day Letter cannot be overstated: it is the last point at which a taxpayer can fight the IRS in a 'pre-payment' court.
2026 Update
In 2026, the threshold for Small Tax Cases (S-Cases) in Tax Court increased to $100,000 per tax year, up from $50,000 under prior law. This allows more taxpayers to use the simplified, less formal S-Case procedures to contest deficiencies under $100,000 without hiring an attorney.
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Related Terms
CDP Hearing (Collection Due Process)
A Collection Due Process hearing is a formal taxpayer right under IRC § 6330 to appeal an IRS lien filing or proposed levy before an independent Appeals Officer, with the option to escalate to U.S. Tax Court if Appeals disagrees.
CAP Hearing (Collection Appeals Program)
A Collection Appeals Program hearing is a faster, less formal IRS appeals process for disputing rejected installment agreements, proposed lien filings, or levies — but unlike a CDP hearing, it does not provide Tax Court access.
Audit Reconsideration
Audit reconsideration is an IRS administrative process that allows a taxpayer to dispute an assessment made from an audit they did not attend, did not agree with, or have new documentation to support.
Taxpayer Advocate Service (TAS)
The Taxpayer Advocate Service is an independent organization within the IRS that helps taxpayers resolve problems with the IRS and recommends systemic changes to prevent future problems.