The study was conducted on 120 patients (76 men and 44 women) affected by idiopathic Parkinson's disease (IPD) responsive to L-dopa and observed for many years. Sixty had clinical onset between the ages of 20-40, representing 10.2% of our PD population; in the others the symptoms began after the 40th birthday. The two groups were matched for sex and length of illness. In all patients a diagnosis of IPD depended on history and clinical and neuroradiological findings. Clinical, pharmacological, evolutive, and epidemiological data were collected on all patients. Thirty-six patients from each group performed motor dexterity tests (reaction time to expected and unexpected stimuli) and cognitive tests (Wechsler Adult Intelligence Scale. Benton, Short tale, and Zazzo's speed and accuracy test). To assess the prevalence of dementia and the severity of psychiatric side effects of L-dopa administration, the 60 patients with early-onset PD were compared with 134 consecutive unselected PD patients. Five percent of early-onset PD patients had a family history of the disorder. Our study showed that early-onset PD does not differ fundamentally from the late-onset form except that the former is characterized by a more rapid establishment of the full-blown parkinsonian clinical picture and deterioration of the therapeutic efficacy of L-dopa, with an earlier appearance of side effects. The results of our neuropsychological investigations suggest that early-onset PD may be a "pure" form of extrapyramidal compromise with exclusively motor manifestations.