By Aug 15, 1985, one hundred ninety-four cases of possible transfusion-associated acquired immunodeficiency syndrome (AIDS) had been reported to the Centers for Disease Control. Cases received their transfusions in 30 states. Infants account for 10% of the cases, suggesting an increased susceptibility to developing AIDS. Investigations one to six years after the transfusions have identified high-risk donors to 47 cases. Of 47 high-risk donors tested, 40 had a reactive serology for human T-cell lymphotropic virus type III/lymphadenopathy-associated virus (HTLV-III/LAV) antibody, including five with no risk for AIDS by history. The HTLV-III/LAV was isolated from 23 of 26 seroreactive high-risk donors, most of whom remained asymptomatic. Blood components that transmitted HTLV-III/LAV included red cells, platelets, plasma, and whole blood. The time from transfusion to diagnosis of AIDS ranged from four to 84 months. The risk of developing AIDS after a blood transfusion has been low and will be lowered further by using both self-deferral and antibody screening.