Reye's syndrome (RS) is generally considered a childhood disease. We report our experience with RS in adults in the metropolitan Milwaukee area. Reye's syndrome was diagnosed in seven 18- to 46-year-old adults. The diagnostic criteria were as follows: viral prodrome followed by vomiting and encephalopathy without focal neurological signs, normal cerebrospinal fluid values, increased levels of serum aminotransferases (transaminase), prolonged prothrombin time, elevated blood ammonia levels, and characteristic microvesicular fatty liver and mitochondrial changes. None of the patients was hypoglycemic. The diagnosis of RS was entertained in 22 but confirmed in only seven patients. In cases of non-Reye's encephalopathy, drug ingestion presented as one of the most difficult differential diagnostic problems, which also included alcohol abuse, collagen vascular disease, and hepatitis B surface antigenemia. Clinical jaundice, distinctly uncommon in RS, was present in only one patient who presented to us in stage V coma. In adults, RS is more difficult to diagnose and should be suspected more frequently in patients with unexplained altered behavior following a viral illness and vomiting. Liver biopsy can be performed safely and is usually mandatory in adults. Patients with RS diagnosed during stage I or II coma and treated experienced an uneventful recovery.