Acute leukemia with megakaryocytic differentiation has been an uncommonly recognized disorder. We used specific monoclonal and polyclonal antibody reagents (HP1–1D antibody and anti-factor VIII antibody, respectively) and an immunocytochemical staining technique to identify the megakaryocytic nature of the leukemic cells of 12 patients who presented with acute leukemia. The leukemic cells of our patients demonstrated the presence of one or both of these platelet- and megakaryocyte-related antigens, but were negative for all of the commonly employed cytochemical and immunocytochemical staining reactions, except for diffuse acid phosphatase activity and granular PAS positivity. Morphologically, the leukemic cells varied in size from 10 to 40 microns in diameter, frequently had cytoplasmic budding, and contained occasional vacuoles and/or peroxidase-negative azurophilic granules. Five patients presented with syndromes of acute myelofibrosis, and seven patients had otherwise unclassifiable acute leukemias, including three patients who had secondary leukemias. Diffuse reticulin myelofibrosis was present in all cases in which it was sought. Chromosomal abnormalities of leukemic cells were found in five cases. Two patients had deficiencies of plasma coagulation factor V. Study of one patient revealed significant platelet dysfunction. When cytoreductive chemotherapy of leukemia was attempted, the observed response was generally poor, with the exceptions of one patient who has remained in complete remission following treatment with etoposide (VP- 16) and a second patient who attained remission following bone marrow transplantation. These cases of acute megakaryoblastic leukemia represented from 3.6% to 9.3% of all acute leukemia cases diagnosed concomitantly in our institution. Acute leukemia with megakaryocytic differentiation may occur more frequently than previously recognized, may present with differing syndromic features, and can be identified by the use of specific antibody reagents and relatively simple immunocytochemical techniques.