Because the prevalence of idiopathic Parkinson's disease (PD) with or without dementia remains controversial, we initiated a population-based investigation in the Washington Heights-Inwood section of New York, NY, so that nearly complete case ascertainment could be achieved. A "registry" was developed for the study, and we advertised in periodicals and on radio and television. Subjects, or their records, were examined by experienced neurologists, and most underwent a battery of neuropsychological tests specifically designed for assessment in this community. All data were reviewed by a team of clinicians to achieve a consensus diagnosis. The crude prevalence of idiopathic PD, with and without dementia, was 99.4 per 100,000, increasing from 2.3 per 100,000 for those younger than 50 years to 1144.9 per 100,000 for those aged 80 years and older. The crude prevalence for PD with dementia alone was 41.1 per 100,000 and also increased with age from zero for those younger than 50 years to 787.1 per 100,000 for those aged 80 years and older. Prevalence ratios were comparable with those of other published population-based studies in similar settings. After standardization, men had PD with and without dementia more frequently than did women. The major difference between patients with and without dementia was a later estimated age at onset of motor manifestations. We conclude that PD is a frequent disorder in the elderly population that affects men and whites more frequently than women and nonwhites. Moreover, dementia in patients with PD is more frequent than previously recognized and is strongly related to the age at onset of motor manifestations.