In 21 parkinsonian patients, we investigated the contents of blood serotonin (5HT) and benzylamine oxidase (BzAO) before and during L-dopa therapy. Then, we studied the patients prospectively, and evaluated the relationship between 5HT and BzAO and the clinical characteristics. The levels of blood 5HT and BzAO varied not only with the treatment, but also with the duration and clinical type of the disease. We postulated that in certain types of Parkinson’s disease, the generalized defect of 5HT and enzyme metabolism might primarily exist, and might influence the efficacy of L-dopa therapy and the prognosis of the diseas