In 520 patients with parkinsonism seen over eight years, 168 (32%) had moderate to marked dementia. Although the demented patients were older than the nondemented patients (70.4 versus 65.5 years), the incidence of dementia in Parkinson's disease (PD) was tenfold higher than among controls (similarly aged spouses of PD patients), and dementia is held to be related more to the disease than to age. Demented patients, in addition to being older, developed PD later, were more severely involved in a shorter time, and responded less well to levodopa. It is suggested that PD with dementia may represent a different disorder from PD without dementia.