19 (51%) of 37 hemophiliac children and adolescents regularly treated with factor VIII and IX concentrates were positive for HTLV III antibodies. The prevalence of HTLV III antibodies was higher in patients with severe hemophilia (64%) requiring frequent administration of concentrates than in patients with mild hemophilia (17%). No patient showed signs of AIDS or AIDS related complex. Immunologic alterations (inverse ratio of helper- and suppressor lymphocytes, elevated immunoglobulins, and elevated total serum proteins) were more often observed in patients requiring frequent administration of concentrates than in patients requiring relatively infrequent administration of concentrates. Since in patients frequently treated with concentrates the prevalence of HTLV III antibodies was also higher, it was not possible to draw any conclusions whether the observed immunologic alterations are due to the HTLV III infection alone or are also induced by the frequent administration of coagulation factor concentrates. No HTLV III positive person was detected in 45 relatives of 18 HTLV III positive hemophiliac children living together in 16 households and actively participating in the care of those children. In contrast, 6 (11%) of 55 relatives living in 21 households with 23 hepatitis-B-positive hemophiliac children were positive for hepatitis B. Our results support the general impression, that the risk to contract hepatitis B seems to be greater than to contract HTLV III from seropositive patients, and should help to facilitate the social integration of HTLV III positive hemophiliac children.