Event-related potentials (ERPs) generated during the performance of visual discrimination tasks were studied in 31 patients with Parkinson's disease, 9 patients with Alzheimer's disease, and 37 normal control subjects. Actively and passively evoked P3 components (P3b and P3a) were respectively identified as the components of the P3 response to infrequent target stimuli and infrequent non-target stimuli. Both the P3a and P3b latencies were significantly prolonged by normal aging. Nine of the Parkinson's disease patients showed a P3b latency above the 95% confidence limit of the age-adjusted regression line based on the normal controls, while only on patient had a prolonged P3a latency. In 6 patients with demented Parkinson's disease, the P3b latency was significantly longer than in 15 age-equivalent normal subjects, although no significant difference was found in the P3a latency. On the other hand, patients with Alzheimer's disease showed significant prolongation of both the P3a and P3b latencies compared to the normal controls. Furthermore, there was a significant difference in P3a latency between patients with demented Parkinson's disease and those with Alzheimer's disease. These results suggest that the automatic processing stage associated with P3a may be less impaired than the attention-controlled processing reflected by P3b in patients with Parkinson's disease, and also indicate that there may be some differences in the changes of cognitive processing caused by Parkinson's disease and Alzheimer's disease.