Both alcohol and HIV (human immunodeficiency virus) can affect the structure and function of the central nervous system. What they have in common is that the white matter of the brain is principally affected, and the direct neuronal injury does not appear to be a consequence of either HIV infection or alcohol use. There is evidence that the cognitive effects of both HIV and alcohol are potentially reversible with treatment or abstinence. There is only limited overlap in terms of the actual brain regions or structures affected, which may account for the observation that the combined effects of alcohol and HIV are largely additive. Although the incidence of alcohol-related Korsakoff amnesia has declined, alcohol use nonetheless is associated with decreased cognitive performance. In the context of HIV infection, very heavy alcohol use in individuals with advanced HIV disease appears to have synergistic effects. Because current studies on the combined effects of alcohol and HIV infection are limited by several issues, including the use of relatively small sample sizes and nonuniform definitions of what constitutes heavy alcohol use, our understanding of the neurological and behavioral complications of this problem still remains incomplete.