Acquired immune deficiency syndrome (AIDS) has been reported in previously healthy homosexual or bisexual males, intravenous drug users, heterosexual men with hemophilia, and Haitians. The finding of heterosexual hemophiliacs with AIDS has raised the possibility of a transmittable blood-borne agent as a cause of this disease. We have found three female half-siblings who had clinical and laboratory evidence for AIDS. All three had evidence of abnormal in vitro cellular immunity; two had chronic active Epstein-Barr virus infection and lymphadenopathy; all three had chronic cutaneous Candida sp infection; and two had Pneumocystis carinii pneumonia. Their mother is a prostitute/drug addict with abnormal T-cell immunity including clinical and laboratory findings of mucocutaneous candidiasis. Histocompatibility typing is consistent with the history of different fathers for each child. Immunologic studies in one child evaluated prospectively from birth were abnormal by 2 months of age. These findings and the clinical histories indicate AIDS and strongly suggest vertical transmission of an agent(s) during the perinatal period.