Isatin, an endogenous monoamine oxidase (MAO) inhibitor, has been found in mammalian tissues. We previously reported that exogenously administered isatin significantly increased acetylcholine (ACh) and dopamine (DA) levels in the rat striatum. In order to elucidate the relationship between isatin and Parkinson's disease, we measured urinary isatin excretions in patients with Parkinson's disease using a newly developed HPLC-UV method. The recovery of this assay was approximately 102.3% at a range from 2 to 50 nmol/ml. The Coefficient of Variance (CV) for the determination at this range was approximately 2.5% for intra-assay and 6.2% for inter-assay, respectively. There was no significant difference in urinary isatin excretion between data from men and women in healthy control. The value in young age group (19-35 years old) was not significantly different compared with that of the older age group (54-84 years old). Urinary isatin levels in patients with Parkinson's disease tended to increase in accordance to the Hoehn and Yahr criteria. This is the first study in which a significant increase in urinary isatin excretion was observed at Stage III, IV and V in patients with Parkinson's disease. Urinary isatin concentrations in drug-treated patients with Parkinson's disease (at Stage I and II) tended to decrease compared with those of patients without medication. These results demonstrated that urinary isatin excretion may serve as an endogenous diagnostic marker for the clinical severity of Parkinson's disease.