We report 12 years' experience with histocompatible, related donor marrow transplantation for 123 patients with acute lymphoblastic leukaemia; 104 > or = second remission. Four regimens were studied: cyclophosphamide (Cy)-+total body irradiation (TBI) (n = 35); Cy+fractionated TBI (n = 45); TBI+high-dose cytarabine (n = 15); and hyperfractionated TBI+Cy (n = 28). 45 patients survive (34 +/- 9%; 95% confidence interval) between 1 and 12.7 years (median 7.8 years) following BMT and 29 +/- 8% survive leukaemia-free. Significantly improved disease-free survival was observed in patients with an initial WBC < 50 x 10(9)/l (P = 0.02). Conditioning regimens tested yielded similar outcomes, though TBI/cytarabine led to greater treatment-associated mortality. Leukaemia relapse was the most frequent cause of failure in 56 +/- 11%; median time of relapse 8 months following BMT, none beyond 2.2 years. Relapse was more frequent with higher WBC, shorter initial remission and previous CNS leukaemia. Acute and/or chronic GVHD was associated with a strong trend (P = 0.06) towards less relapse. Allogeneic BMT may be curative for a substantial fraction of patients with ALL, but additional anti-leukaemic measures beyond these conditioning modifications tested will be required to prevent post-transplant leukaemia recurrence.