One hundred and seven newly diagnosed, untreated patients with Parkinson's disease (PD) were divided into two groups according to their age at reported onset of symptoms. Of these, 79 patients were under age 70 (early-onset) and 28 patients were age 70 and over (late-onset). The group of 50 control subjects comprised spouses, friends of the PD patients, and community volunteers. The patients were participants in a multicenter drug study of Parkinson's disease. Each had received a detailed neurological and neuropsychological assessment in the baseline placebo phases of the study. Thirty-4 patients with early-onset and 12 patients with late-onset were reassessed 3 years after treatment with low-dose levodopa, with bromocriptine, or with a combination of the two drugs. The results of the baseline phase of the study revealed that 8% of the early-onset group and 32% of the late-onset group were classified as demented. The 3-year follow-up revealed that the prevalence of dementia had increased to 17% in the early-onset group and to 83% in the late-onset group. This study confirms that at least two distinct subtypes of Parkinson's disease exist. The subtypes differ both clinically and neuropsychologically. The age at onset of symptoms is a critical determinant of the rate and type of cognitive decline in Parkinson's disease.