Abstract.We tested serum samples from 11 HIV‐seropositive and 32 HIV‐seronegative Italian patients with hemophilia A or B for the presence of lymphocytotoxic antibodies (LCTAs). No patients had the acquired immune deficiency syndrome and all had received over many years heat‐untreated commercial clotting factors. LCTAs directed mainly to B cells, were significantly present in 5/6 HIV‐seropositive patients with hemophilia A and in 5/5 HIV‐seropositive patients with hemophilia B. The presence of LCTAs in HIV‐seronegative hemophiliacs was significantly correlated with both a decrease of T cells and an increase in serum levels of