Since mid-1981 the Centers for Disease Control (CDC) has received reports of more than 1900 cases of the acquired immunodeficiency syndrome. These cases had either Kaposi's sarcoma confirmed by biopsy or a life-threatening opportunistic infection confirmed by biopsy or culture. In January 1982 a hemophiliac with Pneumocystis carinii pneumonia was reported to the CDC, and by July 1982 two other hemophiliacs had developed P. carinii pneumonia. During the next 12 months a total of 22 confirmed cases of the acquired immunodeficiency syndrome occurred in hemophiliacs, 17 in the United States and 5 outside the United States. We report the nature of the epidemic of the acquired immunodeficiency syndrome in hemophiliacs and summarize pertinent clinical aspects.