Thirty-four adult and pediatric hemophilia A and B patients and 50 nonhemophilic members belonging to 28 families were enrolled in August 1984 in a study of human T cell lymphotropic virus type III/lymphadenopathy-associated virus (HTLV-III/LAV) antibody status and T cell subpopulation numbers. All 50 household contacts, including three spouses of LAV antibody-positive adult hemophiliacs, were immunologically normal and serologically negative with respect to HTLV- III/LAV. Based on Western blot serologic testing of blood samples collected intermittently between July 1981 and August 1984 from 33 representative St Louis hemophiliacs studied during the period from 1981 to 1984, the average time since seroconversion was estimated as 20 months. One spouse of a seropositive hemophiliac and 23 parents of 27 seropositive pediatric hemophiliacs assisted regularly with home infusions. These infusion assistants have collectively experienced 44 person-years of concentrate infusion “exposure” without seroconversion. These results suggest that the likelihood for transmission of HTLV- III/LAV from hemophiliacs to persons assisting in their therapy is extremely low.