OBJECTIVE To describe the frequency and compare the clinical characteristics, treatment response, survival and hematologic, immunophenotypic, cytogenetic, and histologic findings in adult patients with acute megakaryoblastic leukemia (AMegL) and megakaryocytic blast crisis of chronic myeloid leukemia (MegBC-CML). METHODS The records of patients with AMegL and MegBC-CML attended in our institution between July 1993 and December 2000 were revised. Megakaryocytic lineage was established by the presence of one or more megakaryocyte/platelet associated antigens (CD41, CD42b, and CD61) in > 20% blast cells. RESULTS In 90 months, 277 patients with acute leukemia were admitted and 25 with chronic myeloid leukemia (CML) in blast crisis (BC) were identified. Twelve of 125 patients (9.6%) with acute myeloid leukemia were AMegL and 32% of cases with CML-BC were MegBC-CML. Leukemic cells of patients with AMegL expressed more frequently CD15 antigen than blast cells of those with MegBC-CML (83% and 37.5%; p < 0.05). In contrast, blast cells expressing myeloperoxidase were present in 50% and 10% of cases with MegBC-CML and AMegL, respectively (p < 0.05). Only one patient in each group obtained remission. Although median survival in patients with AMegL was lower (70 days) than in those with MegBC-CML (175 days) the difference did not reach statistical significance. CONCLUSIONS AMegL and MegBC-CML differ in some clinical and laboratory characteristics and are diseases with poor treatment response and short survival.