To determine the incidence and possible risk factors for dementia in patients with clinically probable Parkinson's disease (PD), a cohort (n = 86) of nondemented patients over 65 years of age with PD fulfilling the PD Brain Bank clinical diagnostic criteria were determined from community records. A similarly aged group of control subjects (n = 102) were recruited from the same area. Both groups were assessed at baseline and approximately 4 years later for cognition, mood, and motor function (PD patients only). The presence and severity of cognitive impairments was based on subject and informant interview, neuropsychological assessment based on the Cambridge Cognitive Examination (CAMCOG) and the application of the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV). At 4 years, 51 (59%) of the PD and 72 (71%) of the control cohort were available for reassessment. Of the PD cohort, 18 (35.3%) had developed dementia and 5 (9.4%) had evidence of mild cognitive impairments. In the control group, 5 (7%) had developed dementia. The incidence of dementia per 1,000 person-years in the PD cohort was 107.1 (95% confidence interval [CI], 59.9-159.8) and in the control group was 17.9 (95% CI, 5.8-31.9). The relative risk of patients developing dementia was 5.1 times that of the controls (95% CI, 2.1-12.5). Increasing age, later age of onset of PD, longer duration of PD symptoms, the presence of hallucinations, and impairment of memory and language function were all predictive factors for the development of dementia (P < 0.05). Dementia was also found to be a significant predictor for institutional placement in the PD group. Compared with similarly aged controls, patients with clinically probable PD have a fivefold-increased risk of developing dementia. This finding has significant implications for successful clinical management of this condition.