Previous research has demonstrated changes in event-related potentials in a variety of cognitive tasks after severe closed head injury. We sought to establish if similar changes were present in patients who had sustained only apparently mild head injury (MHI) by recording event-related potentials in a group of 24 mild head injured and 24 control participants during a three-stimulus auditory target detection task. For this ‘oddball’ task participants were required to press a button every time they heard a rare target tone and to ignore rare novel sounds and frequent non-target tones. Neuropsychological test results indicated that the mild head injured group had mild memory and attention impairments. Analysis of behavioural performance on the three-stimulus ‘oddball’ task showed no difference in reaction times or error rates between the two groups. Target condition N2 deflections appeared to be larger in the mild head injured but peak amplitude measures revealed that this effect was not significant. There were no significant differences in the amplitude or latency of the P3b evoked by target stimuli or the P3a evoked by novel stimuli. However, a putative ‘O-wave’ or ‘reorienting negativity’ following the P3a was more negative in the mild head injured group suggesting increased activation of components of the attention network. These findings lend support to the hypothesis that MHI can cause subtle cognitive impairments that are associated with abnormal allocation of attention resources in the context of normal behavioural performance. (C) 2001 Elsevier Science Ltd. All rights reserved.