Clinical, morphological, immunological, cytogenetical and prognostic features of 84 children under 2 years of age with AML in studies AML-BFM-78, -83 and -87 were retrospectively analysed. There was a high incidence of acute monoblastic leukaemia (FAB M5) (41 patients--49%) and acute megakaryoblastic leukaemia (FAB M7) (study AML-BFM-87: five patients--13%) in this age group. Acute monoblastic leukaemia was associated with hepatosplenomegaly, extramedullary organ manifestations and chromosomal abnormalities involving 11q23. The probability of an 11-year event-free survival of all patients under 2 years of the three studies combined was 39% (SD 6%). While the event-free survival rates of patients aged 2 years and older could be improved in studies AML-BFM-83 and -87 compared with study AML-BFM-78, overall prognosis in children under 2 years in the three consecutive studies remained unchanged. The event-free survival rate of children with acute monoblastic leukaemia in both age groups was comparable (7 yr-EFS (AML-BFM-83 and -87): much less than 2 years--43% (SD 9%), =/much greater than 2 years--33% (SD 9%); P much greater than 0.5). This also applied to other risk groups. In conclusion, taking the high incidence of acute monoblastic and megakaryoblastic leukaemia in children under 2 years into account, no significant differences between children under 2 years or older children concerning response to therapy and overall prognosis could be evaluated.