Forty-three patients with Parkinson's disease (PD) and thirty-seven normal volunteers were subjected to clinical, neuropsychological, neurophysiological (P300 component of the event-related potentials ERP) and radiological (cranial computerized tomographic scanning CCT) evaluation. Intentional memory was more impaired in PD than in normal controls, more so in the demented group of patients, and was related to enlargement of third ventricular size in CCT. While intentional memory was age related in PD patients, perception was age-related in normal controls. Neither global nor specific cognitive functions were related to duration, severity of parkinsonian motor disability, or depression. However, depression in PD was significantly related to parkinsonian motor disability. P300 latency was more prolonged in PD patients than normal controls. P300 parameters of PD patients were not influenced by age, cognitive functions, duration or severity of motor disability, or depression. The reaction time was the only P300 parameter that was age-related in normal controls. Subcortical atrophy as indicated by CCT was more marked in PD and correlated with age in both patients and controls. Subcortical atrophy was significantly related to cognitive functions in PD but not in normal controls. It was concluded that cognitive impairment in PD could be attributed to complex cognitive changes rather than age. It is a disease process, though not directly related to parkinsonian motor disability or depression. PD differed from normal aging as regards the effect of age on the specific cognitive functions, where in PD patients, age was related to intentional memory, yet in normal controls, it was related to perception. Intentional memory deterioration was found to be specific of PD, being related to subcortical atrophy as well as being more pronounced in the demented group of patients.