Essential thrombocythemia (ET), one of the chronic myeloproliferative disorders, is a clonal disorder of multipotent stem cells. Although most patients with ET have a prolonged benign course, a minority of patients may develop a blastic crisis similar to chronic myelogenous leukemia (CML). A case of ET terminating in blastic crisis 8 years after the initial diagnosis is presented. The blast cells were cytochemically and immunophenotypically consistent with the acute myelogenous leukemia with minimal myeloid differentiation subtype of the FAB classification. From the review of the literature on blastic transformation of ET, acute leukemia with an M4 or M7 phenotype occurred more frequently. In addition, three valuable factors to predict the leukemic transformation of ET appear to be karyotypic abnormalities, such as involvement of chromosome 21, previous therapies with a mutagenic potential, and the capability of bone marrow cells to form in vitro spontaneous colonies as in CML.