Survival rates were studied among 1258 children with acute non-lymphocytic leukaemia diagnosed in 1971-88 and included in the population based National Registry of Childhood Tumours. Of the total, 147 (12%) died without receiving treatment. Among the remaining treated children, actuarial five year survival rates were 6% in 1971-4, 15% in 1975-9, 23% in 1980-3, and 40% in 1984-8. Infants aged less than 1 year had a significantly worse prognosis and there was a significant trend towards lower survival rates with increasing white cell count. No independent significant effects on survival were found with sex, French-American-British (FAB) subtype, or the presence or absence of Down's syndrome. Children entered in national trials had a higher survival rate than those who were not entered, and children treated at teaching hospitals had a higher survival rate than those who were treated elsewhere. Among the 535 (43%) children who survived at least one year from diagnosis no factor studied had a significant effect on survival, emphasising the importance of achieving first remission as a determinant of long term survival.