Forty children with Down's syndrome have been identified among 633 patients in the cooperative pediatric BFM studies for acute myelogenous leukemia (AML) since 1987. The following features were characteristic for these patients: (1) a 500-fold higher than expected incidence of megakaryoblastic leukemia (M7) with a peak incidence under 4 years of age; (2) a myelodysplastic prephase with thrombocytopenia lasting for several months up to a few years; (3) frequent involvement of the red cell lineage as suggested by dyserythropoiesis in the bone marrow; (4) coexpression of the lymphoid cell surface antigen CD7 on the leukemic blasts; (5) absence of the translocation t(1;22), instead presence of complete or partial trisomies involving chromosomes 8 and 1; and (6) a good response to chemotherapy. Nearly half of the patients did not receive any or only palliative chemotherapy and subsequently died. In 21 patients treatment according to the AML-BFM protocols was intended, however, with major dose or protocol reductions in six patients. Four died early, 15 (71%) achieved remission. Nine of 11 patients remaining alive for 0.6-6.5 years had received intensive treatment including high-dose cytosine arabinoside (HD-Ara-C). The 5-year survival estimate of 48% +/- 12% was similar to patients without Down's syndrome. In conclusion, these clinical and biological features suggest that M7 leukemia in children with Down's syndrome (M7-Down) is distinct from megakaryoblastic leukemia in other children. Children with Down's syndrome show a favorable outcome if treated adequately. However, overtreatment should be avoided and life-threatening infections pose a particular problem. Therefore, standard-risk AML therapy (including HD-Ara-C) should be considered in children with Down's syndrome and AML.