The role of transplantation in infants with acute lymphoblastic leukemia (ALL) is not defined. We analyzed results of 40 infants diagnosed before age 12 months who received a hematopoietic cell transplant (HCT) between July 1982 and February 2003 in first complete remission (CR1; n = 17), CR2/3 (n = 7), or relapse (n = 16). Patients were conditioned with cyclophosphamide with total body irradiation (n = 39) or busulfan (n = 1). Donors were matched related (n = 8), mismatched related (n = 16), or unrelated (n = 16). Graft-versus-host disease (GVHD) prophylaxis was methotrexate or cyclosporine (n = 7) or methotrexate plus cyclosporine (n = 33). Thirty-nine patients engrafted, 20 developed acute GVHD, and 7 developed chronic GVHD. Sixteen patients relapsed and 7 died of other causes. Patients in CR1 had disease-free survival (DFS) of 76% compared with 45% for CR2/CR3 and 8% for relapse (P < .001). Of 33 patients with cytogenetic data, 26 (79%) had MLL gene rearrangement. Fourteen of these 26 were in CR1 and 11 survive in remission. Outcome was associated with phase of disease, but having the MLL gene was not a factor predictive of outcome. Late effects included growth and other hormone deficiencies. These data demonstrate that infants with ALL and MLL gene have excellent DFS when they received transplants in CR1, and consideration for transplantation in CR1 is warranted.