A patient ingested 15.6 gm acetaminophen and sought medical attention after seven hours. At that time, her plasma acetaminophen level was 217 microng/ml. Treatment with acetylcysteine was begun eight hours after the ingestion. The clinical course of this patient was characterized by a mild illness with a minor elevation of liver enzyme values. A number of factors predicted severe hepatotoxicity for this patient had she not received treatment.