Twenty-six children with acute leukemia were treated with allogeneic marrow transplantation from HLA identical siblings after a conditioning regimen with Cyclophosphamide-total body irradiation (19 patients), Melphalan-total body irradiation (6 patients) or Busulfan-cyclophosphamide (1 patient). Eighteen were transplanted in complete remission (4 with acute non lymphoblastic leukemia in first remission, 14 with acute lymphoblastic leukemia: 6 in first, 6 in second and 2 in subsequent remission): 2 died of cytomegalovirus pneumoniae, 1 relapsed and 15 survive in continuous complete remission from 5 to 42 months after transplantation (median = 22 months). Eight were transplanted in relapse, 7 achieve complete remission, 5 of them relapsed, 1 died of G.V.H. and 1 survives in continuous complete remission 46 months after transplantation. Actuarial analysis shows a disease free survival rate at 3 years of 82% for patients transplanted in remission and 12% for patients transplanted in relapse (p less than 0.01).