L-Dopa was administered to 122 patients with Parkinson's disease over a time period oup to 14 years. The results were summarized below. 1) The akinesia and postural instability scores were significantly improved up to the 9th year. 2) The rigidity and static tremor scores and the Parkinson score were significantly improved up to the 11th year. 3) The Yahr stage was significantly improved up to the 8th year. 4) The time course of Parkinson score was investigated in three groups, Yahr stage I/II group, Yahr stage III group, and Yahr stage IV/V group, taking into account the time interval between the onset of the disease and initiation of L-dopa therapy. No significant difference was found among the three groups. The above results suggested that the long-term prognosis of patients on L-dopa therapy in Japan was better than in Western countries. It was also thought that L-dopa therapy should be instituted at the earliest possible stage of the disease.