This report demonstrates a case of acute megakaryoblastic leukemia evolving in a patient with idiopathic myelofibrosis with myeloid metaplasia. A presumptive diagnosis was made by cytochemical stains, alpha-naphthyl acetate esterase, and alpha-naphthyl butyrate esterase. The diagnosis was established by electron microscopic platelet peroxidase studies of the peripheral blood blast cells and supported by flow cytometry and immunoalkaline phosphatase studies. Specific monoclonal antibodies directed against platelet glycoproteins Ib (6D1) and IIb/IIIa (10E5), which have not been described frequently in analyzing leukemia, were used for flow cytometry and direct immunoalkaline phosphatase technique. Cytogenetic studies demonstrated a deletion of the long arm of chromosome 6 with breakpoints at bands q15 and q23, and ring chromosome 6 (p25, q27). The diagnosis of megakaryoblastic leukemia requires accurate cytochemical stains, platelet peroxidase by electron microscopic examination, and studies employing specific monoclonal antibodies directed against platelets and megakaryoblasts. The exact origin of the circulating megakaryoblasts in these cases is not known and needs additional investigation.