106 patients with acute lymphoblastic leukaemia (ALL) who received bone-marrow transplants from HLA-identical siblings during first or second remission had an actuarial survival rate at 4 years of 43 +/- 12% and an actuarial relapse rate of 32 +/- 12%. 98 patients with more advanced disease had a significantly lower probability of survival (15 +/- 9%) and a significantly higher probability of relapse (67 +/- 14%). Among high-risk patients, those transplanted in first remission had a higher survival probability (55 +/- 22%) than those transplanted in second remission (41 +/- 15%). Relapse rates in the two groups were comparable (28 +/- 24% and 31 +/- 19% respectively). Standard-risk and high-risk patients transplanted in second remission had comparable relapse rates, but there was a trend towards higher survival probability in standard-risk patients. Thus long-term disease-free survival in ALL can be achieved with bone-marrow transplantation. It is not yet certain whether transplants in first remission will result in higher survival rates than transplants in second remission; relapse rates were similar in the two groups.