We report 12 years’experience with histocompatible, related donor marrow transplantation for 123 patients with acute lymphoblastic leukaemia; 104 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 109/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.