A boy with null-cell leukemia received a bone marrow allograft after preparation with chemotherapy and total body irradiation. Cyclosporine A was not administered following transplantation. Renal biopsy performed 6 months after transplantation because of unexplained deterioration of renal function revealed diffuse mesangiolysis and glomerular sclerosis. The significance of this finding is discussed with reference to similar, recently reported cases.