Brain event-related potentials (ERPs) were recorded from nine long-term cannabis users during a complex auditory selective attention task and compared with nine nonuser controls. Stimuli consisted of a random sequence of tones varying in location, pitch and duration. Subjects were instructed to respond to long-duration tones of a particular pitch and location. Cannabis users' task performance was significantly worse than controls. The most striking difference between the ERPs of the two groups was in the greatly enhanced early processing negativity in the user group to short-duration stimuli which matched the target on location only. This is indicative of users engaging in unnecessary pitch processing and thus having difficulty in setting up an accurate focus of attention and in filtering out irrelevant information. The data suggest a dysfunction in the allocation of attentional resources and stimulus evaluation strategies. These results imply that long-term cannabis use may impair the ability to efficiently process information. © 1991.