The immune function was assessed in 22 children, adolescents and young adults with asymptomatic hemophilia, and 15 with thalassemia, in Israel. Five patients with hemophilia and two with thalassemia were found to be severely abnormal, having cutaneous anergy, very low T-helper cells, elevated T- suppressor cells, inverted T-helper/suppressor ratio, reduced response to mitogens and antigens, and nonfunctional NK cells. Four of the five hemophilia patients exhibited profound lymphopenia also. Decreased T-helper and mildly elevated T-suppressor cells with inverted T4/T8 ratio were observed in the hemophiliacs as a group. In the severe group, the reduction in T-helpers and T4/T8 ratio was more pronounced. The thalassemics as a group were found to have increased numbers of T-suppressor cells with decreased T-helper cells in those with intact spleen only. Both groups studied were found to have elevated IgG levels and low natural killer (NK) activity and normal response to mitogens. Cutaneous anergy was found to be a reliable indication for severe T-cell dysfunction and may serve as an early indication of impending AIDS. These results indicate that patients with hemophilia and with heavily hypertransfused thalassemia may be at increased risk of AIDS as they approach adolescence.