Acute lymphoblastic leukaemia (ALL) is rare under 1 year and has a poor prognosis. Only 14 of 48 infants treated on two consecutive MRC UKALL trials remain alive in first remission. Forty infants have subsequently been treated on a protocol incorporating further intensification with an option for high-dose chemotherapy and autologous or allogeneic bone marrow transplantation. The results show no improvement over previous trials largely due to the number of remission deaths; four of nine being associated with toxicity of 5 days of etoposide and cytarabine. Only three of 11 children treated by high-dose chemotherapy and transplantation remain alive in remission. Multivariate analysis of the factors influencing prognosis in all 88 infants showed that only age was significant. Event-free survival was 40% at 5 years for children over 26 weeks but under 10% for younger children. These results show the susceptibility of infants to the toxicity of intensive chemotherapy and do not support the use of short term high-dose chemotherapy alone in the management of infant leukaemia. Despite the unique biological features of infant ALL it appears that these patients also may benefit from longer courses of treatment with a maintenance (continuing) phase.