Plasma beta-phenylethylamine (PEA) levels were determined in 27 patients with Parkinson's disease (PD) in order to evaluate its relation with the severity or clinical course of PD. The plasma PEA concentrations in PD patients were significantly lower (mean +/- SD, 862 +/- 554 pg/ml) than those in the control group (1765 +/- 790 pg/ml) (p < 0.0001). The concentrations of PEA in plasma did not correlate with Hoen and Yahr stage. However, in serial measurements of the PEA of 5 patients in the progressive group of the clinical course of PD, the plasma PEA levels revealed a downward trend after one or two years. On the other hand, in 3 patients of non-progressive group, the plasma PEA remained at the same level. This study suggests that plasma PEA level might reflect nigrostriatal degeneration in patients with PD.