Since early 1985, blood donations in the United States have been screened for antibody to human immunodeficiency virus (HIV). To identify instances of HIV transmission by antibody-negative donations, we investigated 13 persons seropositive for HIV who had received blood from 7 donors who were screened as negative for HIV antibody at the time of donation. Twelve of the 13 recipients had no identifiable risk factors for HIV infection other than the transfusions they had received. On evaluation 8 to 20 months after transfusion, HIV-related illnesses had developed in three recipients, and the acquired immunodeficiency syndrome had developed in one. All seven donors were found to be infected with HIV. On interview, six reported a risk factor for HIV infection, and five had engaged in high-risk activities or had had an illness suggestive of acute retroviral syndrome within the four months preceding their HIV-seronegative donation. Thus, these donors had apparently been infected only recently, and so were negative at the time of blood donation according to available antibody tests. We conclude that there is a small but identifiable risk of HIV infection for recipients of screened blood. To minimize this risk, the reasons for deferral of donation need to be communicated more effectively to blood donors who are at high risk of HIV infection, and new assays that detect HIV infection earlier should be evaluated for their effectiveness in screening donated blood.