The outcome after bone-marrow relapse was assessed in 53 children with acute lymphoblastic leukaemia (ALL). Twenty-five out of 37 children (67%) whose first remission ended in relapse during treatment (group A) achieved a second remission, as did 15 out of 16 (94%) who relapsed after treatment had been stopped (group B). Nevertheless, the median duration of second remission was only 12 weeks in group A and 35 weeks in group B. The median survival from time of relapse was 32 weeks in group A and 75 weeks in group B. It is concluded that marrow relapse is equally serious whether it occurs during treatment or after treatment has been stopped, and that most children with ALL have a single chance of cure at the time of diagnosis.