Between May 1988 and June 2000, 698 children were treated in the Medical Research Council acute myeloid leukemia 10 and 12 trials. The presenting features and outcomes of therapy in these children were compared by age. Although there was no single cutoff in age, younger children were more likely to have intermediate risk and less likely to have favorable cytogenetics (P <.001), and they had a higher incidence of translocations involving chromosome 11q23 (P <.001). The distribution of French-American-British (FAB) types also varied with age; FAB types M5 (P <.001) and M7 (P <.001) were more common in early childhood, whereas older children were more likely to have FAB types M0 (P =.03), M1 (P =.04), M2 (P =.005), and M3 (P <.001). Involvement of the central nervous system at diagnosis was also more common in the youngest children (P =.01). Younger children had more severe diarrhea (P =.002), whereas older children had worse nausea and vomiting (P =.01) after chemotherapy. When adjusted for other important factors, complete remission rates were similar (P =.5) and although there was less resistant disease in younger children (P =.003), this was partially balanced by a slight increase in deaths during induction therapy in younger patients (P =.06). On multivariate analysis overall survival (P =.02), event-free survival (P =.02), and disease-free survival were better (P =.06) in younger children due to a lower relapse rate (P =.02) especially in the bone marrow (P =.02).