Presenting features and natural history were assessed in 48 children with acute lymphoblastic leukaemia less than 2 years of age at diagnosis. Of these, 16 were less than 1 year (group 1) and 32 were between 1 and 2 years (group 2). Results were compared with a group of 348 children between the ages of 2 and 14 years (group 3) diagnosed over the same period. The children in group 1 presented with a higher prevalence of null cell acute lymphoblastic leukaemia, leucocyte counts greater than 100 X 10(9)/l, and hepatosplenomegaly and had a higher central nervous system (CNS) relapse rate and shorter duration of remission than those in the other two groups. Disease free survival and overall survival in group 2 paralleled that of group 3, although children in group 2 had a significantly higher CNS relapse rate. Neurological toxicity resulting from treatment with methotrexate and radiation was common in those under 2 years as a whole. In conclusion, children under 1 year have a particularly poor prognosis, while those between 1 and 2 years have a prognosis similar to that in the older age group. Alternative approaches to CNS prophylaxis are needed to reduce the high prevalence of CNS disease and toxicity.