Objective: To examine the individual and combined impact that traumatic brain injury (TBI) and heavy social use of alcohol have on electrophysiologic correlates of working memory and evaluation of task-relevant information. Design: Case-control study Setting: University hospital brain injury rehabilitation unit. Participants: Forty male volunteers divided into four groups on the basis of their history of TBI and alcohol intake. Subjects with TBI had experienced a severe closed head injury at least 1 year before testing. Main Outcome Measure: Event-related potentials (ERPs) and neuropsychometric tests. Results: Groups showed no significant differences in average age or neuropsychological tests. TBI groups did not differ in time postinjury or on severity measures. Alcohol use measures were significantly greater in the two alcohol groups. N200 latency and P300 amplitude were impaired in heavy social drinkers and in nondrinking subjects with TBI relative to controls, but were significantly impaired in subjects with TBI who were also heavy social drinkers. Conclusion: The results indicate that although alcohol use and TBI independently produce mild alterations in some aspects of late ERP components, the ERP changes are significantly greater when alcohol use and TBI are combined. This study provides evidence that heavy social drinking after TBI has a measurable impact on electrophysiologic correlates of cognition.