An event-related brain potential (ERP) component called mismatch negativity (MMN) is elicited by physically deviant auditory stimuli presented among repetitive, "standard," stimuli. MMN reflects a mismatch process between sensory input from the deviant stimulus and a shortduration neuronal representation developed by the standard stimulus. The MMN amplitude is known to correlate with pitch-discrimination performance. The purpose of the present study was to investigate whether the MMN is different in absolute pitch (AP) possessors and nonpossessors. ERPs were recorded from AP and non-AP groups, which were matched with regard to musical training. It was found that deviant stimuli differing from standard tones by a quartertone or a semitone elicited an MMN irrespective of whether the stimulus was located on (white key/black key) or off the Western musical scale. These results were obtained with both sinusoidal and piano tones. The MMN was larger and earlier when the stimuli were piano tones than when they were sinusoidal tones and when the standard-deviant difference amounted to a semitone rather than a quartertone. However, differences between the groups were not found in auditory information processing reflected by the MMN component of the ERP. In the light of the earlier MMN results showing a close correlation between the MMN and pitch- discrimination accuracy, it might be concluded that pitch discrimination and identification are based on different brain mechanisms. In addition, the differences in the MMN amplitude and latency between sinusoidal and piano tones might be interpreted as suggesting that sensory memory traces, as reflected by the MMN, are capable of storing information of very complex sound structures also.