The acquired immune deficiency syndrome (AIDS) has been reported in hemophiliacs . We examined lymphocyte numbers and functions in 28 patients with hemophilia and 16 healthy adults. Healthy hemophiliacs who had received fewer than 150 commercial factor concentrate (CFC) infusions had normal T-cell numbers, T-cell subsets, and mitogen responses. Healthy hemophiliacs who had received more than 250 CFC infusions had a decreased number of Leu 3+ "helper" T cells, a decreased Leu 3-Leu 2 ratio, and decreased pokeweed mitogen responses. Hemophiliacs who had received greater than 250 CFC infusions and who had persistent lymphadenopathy had decreased Leu 3+ cells, increased Leu 2+ "suppressor" T cells, a decreased Leu 3-Leu 2 ratio, and decreased phytohemagglutinin responses. Abnormal results of T-cell studies were found in 67% of healthy hemophiliacs who had a high degree of CFC exposure and 100% of clinically abnormal hemophiliacs. These findings suggest that hemophiliacs are at risk for the development of the immunologic changes that characterize AIDS and that this risk is related to CFC exposure.