It has been suggested that alterations in the activity of cytochrome P450 monooxygenases may play a role in the pathogenesis of Parkinson's disease, particularly in patients who had onset before the age of 40. We studied the P450-mediated metabolism of acetaminophen to 3-hydroxy-acetaminophen in 26 patients with Parkinson's disease and in 18 control subjects. After subjects ingested 1,000 mg acetaminophen, urine was collected under controlled conditions. Acetaminophen and 3-hydroxy-acetaminophen were measured in the urine using newly developed high-pressure liquid chromatography methods. The ratio of 3-hydroxy-acetaminophen to acetaminophen was calculated for each patient and no significant differences were observed in patients compared with control subjects. Abnormal metabolism was not observed in patients who had onset of Parkinson's disease at or before the age of 40. In addition, no difference in metabolic activity was observed between the patients who were treated with levodopa/carbidopa and those who were not treated. These findings suggest that there are no alterations of P450-mediated metabolism of acetaminophen in patients with Parkinson's disease.