Early reports would suggest that closely matched UD BMT is an adequate substitute for MSD BMT in children with relapsed ALL. Protagonists of BMT might suggest that UD BMT be used in the absence of a MSD in all cases of BM relapse of ALL. However, recent improvements in chemoradiotherapy schedules have reduced the benefits of BMT in terms of overall survival, particularly in children with a long first remission, and a more sensible approach would be to advocate UD BMT in early relapsing aggressive disease, prospectively compare UD BMT to chemotherapy in less aggressive disease, and not utilize UD BMT for low-risk disease. The best prognostic indicators for relapsing disease depend on the site of relapse and duration of first remission. Recommendations for the use of UD BMT in children with relapsed ALL based on these criteria are given in Table 1.