We describe the clinical features of parkinsonism in 25 patients whose age of onset was under 40 years. Among them, 17 patients, whose age of onset was after their 21st birthday, were classified as young onset Parkinson's disease (YOPD), and the remaining 8 whose age of onset was before their 21st birthday were classified as juvenile parkinsonism (JP). Rigidity and akinesia were revealed in all 25 patients. Resting tremor was observed in only 5 patients; 3 in the YOPD group and 2 in the JP group. There were 8 of the 25 patients (32%) who experienced an aching sensation in the leg before or at the onset of the parkinsonian features. In 6 of these 8 cases, the sensory symptoms were on the same side where the clinical manifestations of parkinsonism later developed. In the JP group, 2 patients had right foot dystonia, which improved with levodopa. Diurnal fluctuations in parkinsonian symptoms were found in 9 of the 25 cases. The familial incidence of parkinsonism was higher in the JP group. The parkinsonian disabilities in all 25 cases responded dramatically to levodopa therapy. Unfortunately, 10 cases, 5 in the YOPD group and 5 in the JP group, developed dyskinesia. The longer they took levodopa, the greater the chance of developing dyskinesia. The cumulative percentage of dyskinesia was 100% in the YOPD group and 83% in the JP group by the seventh and fourth year of treatment, respectively. A positive correlation was found between the prevalence of dyskinesia and the duration of treatment in both groups.