An IRS Notice of Deficiency (also called the 90-Day Letter) is the IRS's formal determination that you owe additional tax — and it is the only document that gives you the legal right to petition federal tax disputes to dispute the assessment without paying first. You must file your formal dispute petition within 90 days of the notice date (150 days if abroad); missing this deadline means you must pay the full amount and then sue for a refund in District Court or the Court of Federal Claims. Neil Jesani Tax Resolution's tax professionals evaluate every Notice of Deficiency for filing a petition and have represented clients through settlement and trial. Key Takeaways The IRS Notice of Deficiency (90-day letter) is a formal, statutory notice proposing additional tax liability You have exactly 90 days (150 days if outside the U.S.) to file a formal dispute petition Missing the 90-day deadline permanently eliminates your right to challenge the tax through the formal dispute process without first paying The notice must be sent by certified or registered mail to your last known address Professional representation from Neil Jesani Tax Resolution can protect your rights and help you respond effectively What Is an IRS Notice of Deficiency? An IRS Notice of Deficiency, officially known as a Statutory Notice of Deficiency and commonly referred to as a "90-day letter," is a formal legal document issued by the IRS when it determines that you owe additional federal income tax. This notice is required by Internal Revenue Code Section 6212 before the IRS can assess additional tax against you (with certain limited exceptions). The notice of deficiency is not just another IRS letter — it is a critical legal document that triggers important rights and deadlines. It represents the IRS's final administrative determination of your tax...