Introduction: Mismatch negativity (MMN) is an event-related potential reflecting automatic attention-related information processing marking the detection of auditory change. The bilateral scalp distribution develops by 14 years of age, and is elicited with adult latencies by 17 years. But consistent with reports of continued brain maturation after adolescence we show here that features of the temporal and frontal lobe dipole sources also continue to develop in the third decade of life. This has consequences for studies of the developmental course of MMN anomalies reported in childhood attention-deficit/hyperactivity disorder into adulthood. Methods: Two groups of healthy subjects with mean ages of 17 and 30 years were presented with a 3-tone auditory oddball. The duration-deviant MMN was recorded during attention to a visual discrimination (auditory passive condition) and an active auditory discrimination (between the other tones). Results: 1/ MMN amplitudes were smaller in the older subjects and the MMN lasted longer over the right hemisphere. 2/ Latencies and moments of the 4 dipoles in the temporal and frontal lobes did not distinguish the two subject-groups. 3/ But both temporal lobe sources were located significantly more ventrally and further left in the young adult than in the adolescent subjects. 4/ The left cingular source moved posteriorly and the right inferior frontal source moved antero-medially in the older subjects. Conclusions: A) Brain development in the third decade may cause the two frontal sources to move apart on the rostro-caudal axis but the temporal lobe sources to move left on the lateral and down on the dorsoventral axes. B) Thus special care is necessary in interpreting putative dysfunctional neurobiological changes in developmental attention deficit disorders where as yet unspecified subgroups may show a late developmental lag, partial lag or no lag at all associated with other impairments.