In 1985 acute megakaryoblastic leukemia was included in the FAB classification system of hematological neoplasias with the designation of AML M7. It occurs in all age groups with two peaks in distribution. The one is in adults and the other in children 1 to 3 years of age especially in those with Down's syndrome. The diagnosis of AML M7 requires more than 30% of the nucleated bone marrow cells being megakaryoblasts. The more common types of AML MO-M6 have to be excluded by morphological and cytochemical analysis whereas immunology is needed to exclude ALL. The megakaryocytic nature of the leukemia has to be proven by ultrastructural demonstration of platelet peroxidase or by immunological demonstration of CD61, CD42, CD41 on the surface of the leukemic blasts. Megakaryocytic/megakaryoblastic leukemias show a wide morphologic spectrum. In some instance small cells dominate, clearly showing megakaryocytic differentiation with scant amounts of cytoplasm and with nuclei showing dense chromatin. On the other hand, there are cases with larger cells resembling ALL-L2 blasts with moderate amounts of rather basophilic cytoplasm which in some instances contain azurophilic granules. Cytoplasmic blebs and protrusions are the most prominent feature of many cases. The nuclei of these cells are round with more finely reticulated chromatin and with prominent nucleoli. The megakaryoblastic nature of these cells can be suggested by morphology. However, according to our experience there are cases of c-ALL with the very same morphologic picture. Consequently, immunologic phenotyping of these cases is necessary in any instance. Cytochemistry is of limited diagnostic value in megakaryoblastic leukemias. Usually it is used to exclude the more common types of leukemia.