We have studied habituation of the P3a component of the passive “oddball” auditory event-related potential which reflects automatic processing of a “novel” stimulus in 24 patients suffering from migraine without aura and in 21 healthy volunteers. Three blocks of responses to 160 standard and to 40 novel tones were sequentially averaged at Cz and analyzed for latencies and peak-to-peak amplitudes. Latencies of components N1 and P2 elicited by standard tones and of components N1, P2, N2, and P3a elicited by novel tones were not significantly different between sequential blocks or between subject groups, nor were mean N1-P2 amplitudes. The N2-P3a amplitude tended to be lower in migraine, but not significantly so. The most striking result in migraineurs was a significant potentiation of N2-P3a in successive blocks, contrasting with an habituation in controls. Our previous evoked- and event-related potential studies and the present one suggest that deficient habituation, or even potentiation, represents interictally a fundamental dysfunction of cortical information processing in migraine, which might increase energy demands and play a role in etiopathogenesis.