It is controversial if early onset Parkinson's disease (EOPD) (onset at < 41 years of age) is Parkinson's disease (PD) occurring at a younger age or a different disease. This controversy is due to some clinical and pathological differences between EOPD and PD. Within EOPD, there appear to be two groups namely: young onset Parkinson's disease (YOPD), with onset between 21 and 40 years, and juvenile parkinsonism (JP), with onset at < 20 years. The two major clinical differences between these groups are a higher familial occurrence of PD and dystonia in JP. In this study, we determine if the two groups have the classical features of PD, namely rest tremors, rigidity, bradykinesia, and postural instability, and have a meaningful response to levodopa. Furthermore, we compare their other clinical features, autonomic and cognitive functions, and levels of CSF monoamine metabolites to determine differences between these groups. We observe that all YOPD (100%) and JP (85%) patients had rest tremors. Most of these patients also had a meaningful response to levodopa (YOPD: 72%; JP: 100%). The prevalence of family history of PD was similar, whereas dystonia was more frequent in JP (43%) compared to YOPD (9%). Autonomic symptoms were twice as common in JP (42%) compared to YOPD (17%). However, bedside autonomic functions were abnormal in similar proportions and, like in PD, suggest involvement of parasympathetic nervous system. Cognitive dysfunction does occur but with no difference in severity between the two groups. The difference in number of patients between YOPD and JP groups makes statistical comparison of the occurrence of clinical features like dystonia and autonomic dysfunction difficult.(ABSTRACT TRUNCATED AT 250 WORDS)