• 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.