Cytogenetic, immunologic, and electron microscopic studies were performed on the blast cells of 28 pediatric patients with Down's syndrome, 13 with acute leukemia (DS-AL) and 15 with transient myeloproliferative disorders (DS-TMD). Clonal chromosome abnormalities were found in the cells of all patients with DS-AL but not those with DS-TMD. The younger ages and higher hemoglobin concentrations, platelet counts, and WBC counts of DS-TMD patients provided a clinical contrast with the frankly leukemic cases. Myelodysplastic syndrome, characterized by a small percentage of leukemic blast cells, was observed in 11 of the 13 patients with DS-AL compared with none in the DS-TMD group. Electron microscopy disclosed a positive platelet peroxidase reaction in each of the 11 DS-TMD patients and in nine of the 13 DS-AL patients. Immunologic studies revealed antiplatelet-megakaryocyte antigens on the blast cells of the majority of patients in both study groups. Our findings suggest that the blast cells in cases of DS-AL and DS-TMD arise from cells of the megakaryocytic lineage or from a myeloid progenitor with the capacity for megakaryocytic differentiation. The high risk of the development of AL in patients with DS who are less than 3 years old may be related to increased megakaryocyte proliferation in this age group.