We evaluated the effectiveness of administration of high-dose busulfan (BU), an alkylating agent with substantial myeloablative but negligible immunosuppressive properties, on the engraftment of congenic normal bone marrow and spleen cells (hematopoietic cell transplantation; HCT) in murine galactosylceramidase deficiency (the twitcher mouse), a model of human sphingolipid storage disease. Untreated mice died with extensive demyelbtatioa and failure to thrive at a median age of 40 d (range, 28–47;n= 51). The life-span of twitcher mice given HCT at age 10 d after 100 mg/kg BU was significantly prolonged (median, 94 d; range, 55–140;n= 17); these animals did not develop the hindlimb paralysis seen in untreated twitchers. Histologic examination of twitcher sciatic nerves after HCT showed remyelinated areas and decreased globoid cell inclusions. Animals given HCT after conditioning with BU had both lymphoid and hematopoietic repopulation with donor cells by 90 d after HCT, as documented by presence of donor glucose phosphate isomerase-1A activity in blood, bone marrow, spleen, and lymph nodes. After BU and HCT, galactosylceramidase activity in livers and spleens of twitcher mice reached 45 and 80% of control, respectively; only modest elevations were observed in kidneys and lymph nodes. Hydrolase activity rose to 20% in the brains and exceeded control values in the sciatic nerves of transplanted twitcher mice, indicating entry of at least some donor-derived cells and/ or hydrolase across the blood-brain and blood-nerve barriers after BU and HCT. These observations suggest that, in gene-replacement therapy for human lysosomal storage diseases, administration of BU alone may be sufficient for sustained engraftment of autologous marrow cells into which the specific hydrolase gene has been inserted. (Pediatr Res29: 302–305, 1991)