A child with severe neutrophil dysfunction and intractable infections received bone marrow transplants from histocompatible siblings. After a first transplant preceded by cyclophosphamide (CY), antithymocyte serum (ATS) and procarbazine (PCB) preconditioning, there was no evidence for engraftment and autologous marrow function rapidly returned. Cell mediated lysis showed no evidence of patient sensitization against the marrow donor suggesting that graft rejection did not cause the transplant failure. A second transplant was performed utilizing another matched sibling donor. Total body irradiation was added to CY, ATS, and PCB for preconditioning after in vitro studies of the colony forming capacity (CFUsub(c)) of the patient's marrow cells showed normal sensitivity to radiation. Full engraftment ensued with correction of granulocyte function abnormalities. The patient eventually died of intractable pulmonary disease. Experience with this child suggests that cyclophosphamide alone may be insufficient preparation for marrow transplantation in some patients with non-neoplastic hematologic disorders. Experimental and clinical data supporting this contention are reviewed.