To evaluate the consequences of receiving human immunodeficiency virus type 1 (HIV-1)-seropositive blood, 90 HIV-1-seronegative recipients of HIV-1-seropositive blood (case patients) and 90 HIV-1-seronegative recipients of HIV-1-seronegative blood, matched for age, sex, number of transfusions, diagnosis, and severity of illness (controls), were followed for 12 months after transfusion at Mama Yemo Hospital in Kinshasa, Zaire. Of case patients and controls, 72% were children transfused for anemia caused by malaria. Of the 46 case patients case patients alive 6 months after transfusion and for whom HIV-1 serologic results were obtained, 44 (96%) had seroconverted. Significantly more case patients (47%) than controls (16%) died within 1 year after transfusion (P less than .001). In the first 3 months after transfusion, fatigue, diarrhea, fever, cough, pruritus, pallor, oral candidiasis, polyadenopathy, hepatosplenomegaly, and rhinorrhea were observed more often among seroconverters than controls (P less than .04). Six percent of case patients and no controls had developed clinical AIDS after 12 months of follow-up. These findings underscore the urgent need for appropriate HIV screening facilities in transfusion centers worldwide.