Bone-marrow transplantation has been used in patients with acute lymphoblastic leukaemia (ALL) thought to be at high risk of relapse if managed with chemotherapy. Data from 444 ALL patients with one or more high-risk features at diagnosis were analysed to evaluate outcome after HLA-identical bone-marrow transplantation during first or during second remission. The 4-year actuarial probability of leukaemia-free survival was 45% (95% confidence interval 36-54%) for transplants in first remission compared with 22% (15-29%) for those in second remission (p less than 0.0002). The 4-year probabilities of relapse were 26% (14-38%) and 56% (45-67%) respectively (p less than 0.0001). For high-risk ALL, transplantation in first remission had clearly superior results to transplantation in second remission. Further studies are needed to determine whether patients with high-risk ALL should receive transplants during first remission or should initially receive chemotherapy, with transplantation being reserved for patients who relapse.