The human immunodeficiency virus (HIV) is the agent that causes acquired immunodeficiency syndrome (AIDS). HIV is transmitted through sexual contact with an infected individual, through exchange of infected blood or blood products, or to a newborn from an infected mother. Eventually, HIV destroys the body's immune system, mainly by impairing a class of white blood cells whose regulatory activities are essential for immune protection. Alcohol in the body can impair normal immune responses that protect the body from disease. Chronic alcohol consumption has been shown to reduce the number of infection- fighting white blood cells in laboratory animals and in humans. These effects could mean that drinking may increase vulnerability to HIV infection among people exposed to the virus and that among people who are already HIV infected, alcohol-induced immunosuppression might add to HIV-induced immunosuppression and speed the onset or exacerbate the pathology of AIDS-related illness. Sexual practices considered to be high-risk for acquiring HIV from an infected individual include vaginal or anal intercourse without a condom; other sexual practices that facilitate exchange of blood, semen, or other body secretions; and unprotected sexual activities with multiple partners. Alcohol's relationship to high-risk sexual behavior may be explained in two ways. First, alcohol use may be a marker for a risk-taking temperament. Second, alcohol may influence high-risk behaviors at specific sexual encounters by affecting judgment and disinhibiting socially learned restraints. 36 references