A 21-month-old boy with the Wiskott-Aldrich syndrome conditioned with cyclophosphamide and dimethyl myleran received bone marrow from an HLA-matched sibling. Complete hematological chimerism was achieved. During the first 3 months after transplantation, in vitro B cell function, measured by a direct plaque assay, was abnormal, T cell helper activity impaired, and suppressor T cell function was excessive. These abnormalities resolved gradually over 16 months. Antibody responses to the T-dependent antigen, bacteriophage phi X174, were initially low, then became normal; antibody responses to keyhole limpet hemocyanin (KLH) and to 4 of 12 type-specific pneumococcal polysaccharide antigens were adequate when studied 9 months after transplantation. The clinical response was excellent: the patient has been free of infection, no longer has a bleeding tendency, and has shown normal growth and development.