The purpose was to determine if unmedicated, mildly affected patients with Parkinson's disease exhibited abnormality in the P3 component of the event-related potential, and whether such abnormality differed between younger and older patients. The study evaluated 10 younger (mean age = 43.7 years) and 10 older (mean age = 64.4 years) unmedicated patient volunteers diagnosed with idiopathic PD during the past 4 years and equal numbers of age-, gender-, and education-matched controls. The auditory oddball P3 was recorded, and P3 peak amplitude, peak latency, and the component score derived from principal components analysis were analysed. Neuropsychological measures focusing on frontal lobe and memory function were obtained. Although patients did not show neuropsychological deficits, they had significantly enlarged P3 amplitude measured at Cz (p < 0.01) or Pz (p < 0.01). The P3 amplitude abnormality among patients was not affected by age. Patient P3 latency was not prolonged. The results indicate that P3 amplitude may more sensitive than neuropsychological measures for detecting subtle brain dysfunction occurring early in PD. This measure has possible utility for detecting and tracking early disease. It is hypothesized that enlarged P3 amplitude reflects abnormality in use of attentional resources to compensate for brain dysfunction.