A reasonable cause defense is a legal argument that asks the IRS or a court to remove a tax penalty because the taxpayer exercised ordinary business care and prudence but still could not comply.

In plain terms, it is not enough to say a mistake happened. The taxpayer must show a credible reason for filing late, paying late, or reporting something incorrectly despite making a genuine effort to follow the law. Common examples include serious illness, a natural disaster, destroyed records, or relying on incorrect professional advice in limited situations. Forgetfulness, lack of money by itself, or not knowing the rules usually is not enough. The IRS recognizes reasonable cause for some penalties under Treasury regulations and Internal Revenue Manual guidance, but the facts matter more than labels.

This defense matters because tax penalties can grow fast, especially when paired with interest and collection pressure. A successful reasonable cause argument can reduce or eliminate failure-to-file penalties, failure-to-pay penalties, and some accuracy-related penalties. That can make a big difference in an audit, appeal, or installment agreement.

It can also affect an injury-related case. If someone receives a settlement, misses a tax deadline because of hospitalization, trauma, or disrupted records, those facts may support penalty relief even if some tax is still owed. Good documentation - medical records, letters, timelines, and proof of efforts to comply - is usually what makes the defense work.