An estimate of the incidence of dementia in idiopathic Parkinson's disease. The proportion of patients with idiopathic Parkinson's disease (PD) who are considered demented ranges from 10% to 15%. Because dementia may affect survival in PD, the incidence rate of dementia, rather than proportion, would be a more accurate measure of disease frequency. We previously estimated the proportion of patients with PD and dementia to be 10.9% from the records of a cohort with the idiopathic form of PD in a major medical center. We reviewed the clinical records of this cohort after 4 years and 9 months to estimate the incidence rate of dementia. We identified 65 new cases of dementia from the 249 patient-records available. Using the number of person-years of follow-up for each case as the denominator, we estimated the overall incidence rate to be 69 per 1,000 person-years of observation. The mean age of this cohort was 71.4 years. The cumulative incidence of dementia increased with age. By 85 years of age, over 65% of the surviving members of the cohort were demented. The age-specific incidence rates for dementia in this cohort of PD were significantly greater than for a similarly aged cohort of healthy elderly people. The age-specific standard morbidity ratios indicated that, compared with people of similar ages, patients with PD have the highest increase in risk for dementia between ages 65 and 75. 