We followed up, after 3 1/2 years, a whole population cohort of 249 patients with Parkinson's disease (PD) 1st examined in 1983 to 1984. Of the survivors, 23.6% qualified for a DSM III-R diagnosis of dementia. In univariate tests, age, certain items of the Webster scale, the Hoehn and Yahr scale, a 10-question mental status questionnaire, and a history of smoking were associated with a diagnosis of dementia 3 1/2 years later. Logistic regression with DSM III-R diagnosis (demented versus nondemented) as the dependent variable, and age and symptom scales for PD as predictor variables, revealed that PD symptoms predicted dementia even after controlling for age. We conclude that dementia is probably more common in PD patients than would be expected in the general population and is associated with the severity of PD symptoms and signs independently of age.