There has been some debate about abnormalities in visual evoked potentials (VEPs) in Parkinson's disease (PD). To elucidate the mechanism underlying abnormal VEPs, we investigated the relationship between pattern-reversal VEPs elicited by large checks and mental functions in PD patients (n = 32), as compared with VEPs of age-matched control subjects (n = 22). The PD patients were divided into two groups: PD without dementia (nD-PD; n = 21) and PD with dementia (D-PD; n = 11). All patients but five of the nD-PD patients were being treated with anti-parkinsonian drugs. The D-PD patients showed significantly prolonged P100 latencies compared with both the nD-PD patients and controls (p < 0.01 and p < 0.01, respectively). The PD patients treated with levodopa had significantly longer P100 latencies than the other PD patients. In PD patients, the P100 latency correlated significantly with illness duration (p < 0.05). There was also a significant negative correlation with P100 latency and Mini-Mental State Examination (MMSE) score (p < 0.05). The MMSE score did not correlate with illness duration in PD patients. These findings suggest that the VEPs abnormality elicited by large checks is related to dementia independent of progression of the illness, and that a nondopaminergic neurotransmitter system may play a role in the development of VEPs delay elicited by large checks.