Deviant tones embedded in a sequence of standard tones elicit an event-related potential (ERP) component called the mismatch negativity (MMN), which reflects automatic stimulus change detection in the human auditory system. To determine whether stimulus change detection is impaired in Parkinson's disease (PD), we recorded ERPs in 13 non-demented patients with PD and in 11 age-matched healthy control subjects both when tones were attended and when they were unattended. The difference area between deviant and standard ERPs was used to evaluate the MMN in the 'unattended' condition. The MMN was significantly smaller in patients with PD than in controls. In the attended condition, the amplitudes of both N1 and the N2 complex were smaller in the patient group than in controls. MMN attenuation suggests that PD patients have impaired automatic stimulus change detection compared with healthy controls. This might be caused by dopamine deficiency in PD.