A subgroup of children with ALL remains at high risk of relapse despite the administration of intensive chemotherapeutic protocols and may benefit from allogeneic BMT. The cytoreductive regimen used most often combines TBI with cyclophosphamide. Nevertheless, miscellaneous long-term sequelae have been consequent upon radiotherapy, especially in young children. This retrospective multicentric study analyzes the outcome of children with ALL under 4 years of age receiving an HLA-genoidentical BMT following a radiation-free preparative regimen. A busulfan-based regimen with cyclophosphamide or melphalan +/- etoposide +/- cytarabine was given to 21 children (median age: 28 months, range 6-48). Sixteen patients with initial poor prognostic factors were transplanted in first complete response (CR) and five patients in relapse or second CR. With a median follow-up of 47 months, the results show an overall 4-year DFS of 61.1%. Leukemic recurrence was observed in eight patients. The preparative regimen was well-tolerated and there were no transplant-related deaths. A busulfan-based BMT preparative regimen may be a therapeutic alternative to TBI-containing regimens in young children. Efforts are currently aimed at reducing the relapse rate in these children by optimizing the tumoricidal potential of chemotherapy and the graft-versus-leukemia effect of allogeneic BMT.