Variations in the activities of xenobiotic metabolizing liver enzymes may be involved in the pathophysiology of diseases, including Parkinson's disease. We therefore studied the activity of the debrisoquine metabolizing enzyme in 97 patients with newly diagnosed Parkinson's disease. The urine debrisoquine metabolic ratios (MR) of the patients were compared with a group of 176 healthy subjects. There were 4 poor metabolizers (4.1%) among the parkinsonians. This proportion did not differ from that found in the group of healthy subjects (51%). In contrast to earlier finding, the parkinsonian poor metabolizers (PM) had the onset of the disease later than the parkinsonian extensive metabolizers (EM). In the parkinsonian patients, it was observed that the excretion of debrisoquine and 4-OH-debrisoquine into urine correlated inversely with the actual age and age at disease onset. Our results indicate that in patients with Parkinson's disease, debrisoquine hydroxylation is comparable with healthy subjects.