Acquired immune deficiency syndrome (AIDS) is a newly recognized disease of unknown etiology, characterized by deregulation of the cell-mediated immune function system and manifested by opportunistic infections, unusual neoplasms (particularly Kaposi's sarcoma) in previously healthy persons. Male homosexuals, drug addicts, Haitian immigrants, and hemophiliacs constitute the group at high risk of having AIDS. The disease is probably caused by an as yet unidentified agent that is transmitted from person to person via sexual contact, blood, or blood products. Available therapy cannot reverse the underlying immune defect. Mortality at two years exceeds 70%. This article reviews the current state of our knowledge about AIDS. Selected aspects of the epidemiology, diagnosis, treatment, immunology, and etiology of the disease are discussed.