Notice of Deficiency

A Notice of Deficiency is a formal notice issued by the Internal Revenue Service (IRS) when it determines that a taxpayer has underreported income or owes additional taxes. It is often called a “90-day letter” because the taxpayer generally has 90 days (150 days if addressed to a person outside the United States) from the mailing date to petition the United States Tax Court for a redetermination of the deficiency before the IRS may assess the tax.

26 U.S. Code § 6212 authorizes the IRS to issue the notice, and 26 U.S. Code § 6213 grants taxpayers the right to petition the Tax Court before assessment or collection. The notice must state the amount of the deficiency and explain the basis for the IRS’s determination. Courts have emphasized that the notice serves as the taxpayer’s gateway to judicial review. See, e.g., Farhy v. Commissioner, 100 F.4th 223 (D.C. Cir. 2024) (holding that a Notice of Deficiency is required to provide taxpayers an opportunity for judicial review before assessment).

[Last reviewed in September of 2025 by the Wex Definitions Team] 

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