Cognitive deterioration is not an initial symptom of Parkinson’s disease but occurs over the course of the disease with the incidence of dementia. The P300 component of event-related potentials appears to be sensitive to cognitive impairment, and has been used to examine cognitive functions in Parkinson’s disease. Although many studies have shown that P300 latency and amplitude are correlated with cognitive functions, there is a sufficient amount of controversial results. The authors have reviewed the results and suggested that the method should be used only in the combination with clinical and neuropsychological examinations. Further studies are needed.