PIP: This review article on acquired immunodeficiency syndrome (AIDS) covers its epidemiology, clinical spectrum, and etiology. Despite intensive efforts, the cause and pathogenesis of this syndrome remain unknown and effective therapy is not yet available. In addition, the clinical presentation of AIDS is variable, ranging from generalized lymphadenopathy to dermatologic lesions, pneumonias, enteritis, ophthalmologic lesions, disseminated disease, malignancies, neurologic conditions, paresthesias, radiculopathies, seizures, and psychiatric manifestations. The population at risk for AIDS icludes homosexual or bisexual men (71% of cases), intravenous drug abusers (17%), those born in Haiti who recently migrated to the US (5%), and hemophiliacs (1%). 6% of cases reported to the US Centers for Disease Control (CDC) fall in none of these categories. 47% of AIDS patients are 30-39 years of age at time of diagnosis. The CDC's strict surveillance definition of AIDS represents only 1 end of the clinical spectrum and could result in an underestimate of the size of the problem. It ignores the prodromal or milder forms of AIDS-related illnesses that have less clearly defined clinical courses. The persistent, irreversible immunosuppression characteristic of this syndrome creates a complex management problem for physicians, but the early recognition of AIDS by emergency and primary care physicians may improve the grim prognosis for AIDS victims.