We evaluated the concentrations of free and total serotonin (5-hydroxytryptamine, 5-HT) and its related substances in the cerebrospinal fluid from patients with Parkinson's disease (PD). The concentrations of total 5-HT, 5-hydroxytryptophan, kynurenine and 3-hydroxykynurenine decreased significantly in PD patients compared with controls. The concentration of total 5-HT had significant negative correlations with Hoehn and Yahr's stages, the severity of rigidity, akinesia and gait freezing; the correlation with gait freezing was most conspicuous.