Recent studies indicate a high prevalence of seropositivity to the lymphadenopathy-associated virus/human T-lymphotropic virus (type III) among individuals with hemophilia exposed to clotting factor concentrates prepared from large donor pools. The peripheral blood lymphocytes of 19 young seropositive patients with inherited bleeding disorders were examined for the presence of this virus by coculture with phytohemagglutinin-stimulated lymphocytes. Viral isolates were obtained from six of 19 patients. While none of these patients have developed the acquired immunodeficiency syndrome (AIDS) or AIDS-related complex, five of them had lymphadenopathy in two noncontiguous areas, and two showed clinically symptomatic enlarged tonsils and adenoids. Of the 13 patients in whom virus was not demonstrated, five were judged clinically normal and five had mild lymphadenopathy in one anatomical area. These results suggest that as many as 33% of hemophiliacs (six of 19 patients studied), who have circulating antibodies to mature viral proteins, have viral-infected peripheral blood lymphocytes capable of infecting other lymphocytes in vitro.