We investigated the use of a new conditioning regimen followed by allogeneic bone marrow transplantation (BMT) for treating children with acute lymphoblastic leukaemia (ALL) after relapse within 6 months of the completion of therapy. One hundred and sixteen children with acute lymphoblastic leukaemia in second or subsequent complete remission (CR) underwent allogeneic bone marrow transplantation from HLA-identical siblings after a preparative regimen comprising total body irradiation (TBI), high-dose cytosine arabinoside and melphalan (TAM regimen). The Kaplan-Meier product-limit estimate (mean +/- SE) of disease-free survival (DFS) at 7 years was 59.5 +/- 9% (95% confidence interval). The estimated chance of relapse was 22.5 +/- 15% with a median follow-up of 88.5 months (range 51-132). 26 patients (22.4%) died with no evidence of recurrent leukaemia, mainly from interstitial pneumonitis, veno-occlusive disease or acute graft-versus-host disease (GVHD). Three factors significantly affected DFS: acute GVHD. site of relapse and, for children in second remission after a marrow relapse, the disease status at the time of transplantation. The DFS were 59.02 +/- 12.6%, 37.5 +/- 19.8% and 774 +/- 15% among patients in CR2 after a marrow relapse, in CR3 or in untreated partial marrow relapse, and in CR2 after an isolated CNS relapse, respectively. The lowest DFS was seen in children with acute GVHD grades 3-4. Two significant factors were associated with relapse: the marrow status at the time of transplantation and chronic GVHD. The relapse rate was lower among children in CR2 or with chronic GVHD. We conclude that transplantation after the TAM regimen is an effective therapy for this population with acceptable toxicity, particularly for children in second remission after a very early marrow relapse, or those with early isolated CNS involvement.