During the last two years a new syndrome has been recognized which involves a severe acquired cellular immunodeficiency. More than 1300 cases have already been identified, mainly in the United States. Clinically the patients present with opportunistic infections, especially Pneumocystis carinii pneumonia, and/or tumors (often a Kaposi sarcoma). Frequently this is preceded by a prodromal phase with fever, weight loss and lymphadenopathies. The overall mortality is over 40% and may reach 100%. Groups at risk for the syndrome are male homosexuals, i.v. drug abusers. Haitians, possibly some Africans and patients who have received blood products. These epidemiologic data suggest that the syndrome is probably due to a bloodtransmitted infectious agent.