The acquired immune deficiency syndrome (AIDS) is a devastating new illness which appears to be sexually and parenterally transmissible. AIDS was first described in the male homosexual community; however, the disease has more recently been described among intravenous drug abusers, Haitians, hemophiliacs, and others. The etiologic agent is unknown. AIDS may represent an infection by a previously undescribed organism, a mutant of a known microorganism, or a multifactorial combination of environmental, immunologic, and genetic factors. As a consequence of the disease's seemingly irreversible ablation of the cell-mediated immune system, AIDS victims succumb to a variety of infections and/or unusual neoplasms. In its fully developed form, mortality approaches 100%. At autopsy the gross and microscopic pathology of the syndrome can be divided into three general categories: 1) morphologic manifestations of profound lymphoid depletion; 2) infections, usually with mixed opportunistic pathogens; and 3) unusual neoplasms, most frequently Kaposi's sarcoma or high-grade lymphomas.