Forty-three neurologically and psychiatrically assessed patients with idiopathic Parkinson's disease (PD) underwent detailed cognitive assessment. Cognitive deficits typical of senile dementia of Alzheimer's type (SDAT) were found in 7% but the majority showed definite impairments not typical of SDAT. Cognitive impairment was significantly more likely in those with more severe PD symptoms. There was substantial agreement between psychiatric diagnosis and psychological picture of SDAT and some links were found between other diagnostic categories and nature of cognitive functioning. However, cognitive deficits were also found In two-thirds of patients with no psychiatric diagnosis.