A whole population cohort of 157 patients with idiopathic Parkinsonism, most of whom had previously been clinically examined by Mutch (1986a), were assessed to determine prevalence figures for dementia and examine the relationship between dementia, cognitive impairment and Parkinsonian signs. Dementia according to DSM-III-R criteria was diagnosed in 23.3% of all patients (95% confidence interval: 17.1 to 32.4%). Dementia and cognitive impairment were associated with overall measures of Parkinsonian impairment and rigidity, but not tremor, even after controlling for age, sex and education.