The clinical, hematologic, and immunophenotypic features in 20 patients with Down's syndrome (DS) and acute leukemia were analyzed. Of the 20 patients, all 14 patients who were 3 years old and less were diagnosed as having acute megakaryoblastic leukemia (AMKL) by use of platelet- specific monoclonal antibodies and platelet peroxidase (PPO) reaction in electron microscopy. They were characterized by the presence of bone marrow fibrosis, having a history of myelodysplastic syndrome (MDS) and a poor response to chemotherapy. Only one patient has remained in continuous complete remission for more than 1 year. Acute leukemia in six patients who were older than 4 years was classified as common acute lymphoblastic leukemia antigen (CALLA)-positive acute lymphoblastic leukemia (ALL). In one of six patients classified as ALL, the leukemic blasts simultaneously expressed myeloid-associated surface antigens. All six patients achieved a complete remission and have remained in continuous complete remission and have remained in continuous complete remission from 10 to 52 months from the initial diagnosis. Although it has been suggested that the distribution of types of acute leukemia in patients with DS is similar to that in normal children, the present study shows that the distribution of acute leukemia types is quite different from that in patients without Down's syndrome.