Seventeen non-demented patients with idiopathic Parkinson's disease were compared with an age and sex matched control group on an auditory oddball task. Low probability target tones were either counted silently or responded to by a button press. N1 amplitude in the Parkinson group was attenuated to both target and non-target tones suggesting an impairment in early information processing. In contrast amplitudes of P2, N2 and P3 did not differentiate patients from controls. Several peak latencies (P2, N2 and P3) were increased in the Parkinson group when targets were counted, whereas only N2 was delayed when targets were identified by a button press. The longer N2 latency is suggestive of an increase in the time needed to categorize stimuli. The amplitude and latency changes of early ERP components provide evidence for impairment in early information processing in Parkinson's disease.