It has frequently been reported that in so-called oddball tasks, children with attention-deficit and hyperactivity disorder (ADDH) show small P3 peaks of the event-related potential (ERP) in response to "targets" (task-relevant deviant stimuli) than normal children. It is not clear, however, whether this smaller P3 is due to abnormal processing of infrequent stimuli per se and/or of task-relevant stimuli and whether it is preceded by abnormalities in earlier peaks, especially those thought to be related to automatic deviancy detection [mismatch negativity (MMN) in the auditory modality and P2N2 in the visual modality]. ERPs of ADDH and normal children in response to visual and auditory stimuli were studied in a condition without task relevance as well as in a task-relevant condition. ADDH children showed smaller P3 amplitudes and (marginally) smaller MMN to auditory deviant stimuli, irrespective of task relevance, so smaller P3s in ADDH children are due to stimulus deviancy per se. In the visual modality the P3 effect failed to reach significance. Because the smaller P3s were also found in a condition not requiring task-related motivation, recent motivational interpretations of differences with normal children are not supported. ADDH children also showed smaller P1 amplitudes than normal children to all stimuli except visual novels. The ERP differences were unrelated to performance, since both groups performed equally well.