Immunologic abnormalities qualitatively similar to those seen in acquired immunodeficiency syndrome (AIDS), including a low helper/suppressor lymphocyte ratio and low natural killer (NK) activity, have been observed in many hemophiliacs receiving clotting factor concentrates. To determine whether these changes also occur after repeated blood transfusion, we measured helper/suppressor (T4/T8) ratios and NK activity in four groups of test subjects: (A) 30 subjects with sickle cell anemia (SCA) receiving monthly transfusions, (B) 30 nontransfused sickle cell anemia subjects, (C) 87 individuals with hemophilia or severe von Willebrand's disease, and (D) 30 normal controls. Like the hemophiliacs, transfused SCA subjects had low T4/T8 ratios and low NK activity compared to controls. Nontransfused SCA subjects had normal values. These findings suggest that a modest decrease in T4/T8 ratio and NK activity may be part of the normal immune response to repeated transfusion.