We recorded event-related brain potentials (ERPs) from 2 groups of human immunodeficiency virus (HIV)-positive men with no physical illness or neurologic involvement: 9 asymptomatic (AS +) and 9 classified as having either acquired immunodeficiency syndrome (AIDS; 7) or AIDS-related complex (ARC; 2). In separate choice reaction time tasks, the subjects pressed buttons to randomly presented auditory or visual stimuli at probabilities of either 20/80 or 50/50. There were no group differences on any of the neuropsychological tests commonly used in screening batteries for HIV patients. In contrast, reduced P300 amplitudes and increased P300 latencies occurred in ARC/AIDS patients in response to both auditory and visual stimuli, while in AS+ patients such alterations occurred only in the visual modality. Significant delays in P2 latency were found only in the auditory modality and then only in ARC/AIDS patients. ARC/AIDS patients alone showed delayed response times, and only in the auditory modality. The P300 results demonstrate alterations in stimulus evaluation and processing speed in the earliest stages of HIV disease, even before cognitive deficits can be detected by more traditional measures.