Since that time, AIDS was reported in patients with hemophilia and in others receiving blood transfusions. The NHLBI was delegated responsibility for studies to elucidate, through a systematic prospective study, the possible association of blood product use with AIDS. The incidence of transfusion-acquired AIDS was so low as to preclude a general prospective study of transfusion recipients to determine the overall incidence of the disease. By focusing on patient groups that appeared to have a higher likelihood for developing AIDS because they required chronic, repeated transfusion therapy, information about the epidemiology of the disorder could be more readily obtained. Continuing medical evaluation of these patients from apparent high-risk groups in a prospective fashion allowed identification of significant emerging physiologic alterations and to follow the time-course of pathologic developments.