Renal disease has not been considered a major late complication of bone marrow transplantation. Of 31 evaluable pediatric patients undergoing allogeneic or autologous bone marrow transplantation for neuroblastoma or acute lymphoblastic leukemia, 14 developed a hemolytic anemia, microscopic hematuria, and renal insufficiency at a median of 5 months (range, 3 to 7 months) posttransplant. Renal biopsies were performed in two patients at the onset of kidney disease and showed mesangiolysis with intraglomerular capillary aneurysm formation, mesangial proliferation, and focal thickening and splitting of the glomerular basement membranes. The clinical presentation, time to onset of renal disease, and biopsy material are consistent with a diagnosis of radiation nephritis, a previously uncommon finding in this patient group. The high incidence of this syndrome in the current report may have been due to the combination of intensive chemotherapy and total-body irradiation in the conditioning regimens.