Recently, a new epidemic illness, the acquired immunodeficiency syndrome, has dramatically emerged in the United States, Europe, and Haiti. The syndrome represents an unprecedented epidemic form of immunodeficiency involving prominent defects of the T-lymphocyte arm of the immune system. Pneumocystis carinii pneumonia, other opportunistic infections, and the previously rare cancer, Kaposi's sarcoma, are the most conspicuous illnesses that have this profound state of immune compromise as their underlying basis. Two years after the onset of clinical illness the case-fatality rate may exceed 90%. A steadily growing body of epidemiologic evidence indicates an infectious (probably viral) cause of the immunodeficiency although the responsible agent(s) remains obscure. Critical issues surrounding the diagnosis, screening of blood products, treatment of complicating infections and cancers, and prognosis for immunologic recovery in affected persons are unresolved. The identification of the cause of acquired immunodeficiency syndrome and the institution of effective preventive measures require the urgent attention of the medical and scientific community worldwide.