Three cases of megakaryoblastic transformation of chronic granulocytic leukemia (CGL) are reported. In each case, the leukemic transformation had morphologic features suggesting megakaryocytic differentiation. This was confirmed by positive immunostaining with a monoclonal antibody (HP1-1D) specific for platelet and megakaryocyte glycoprotein IIb/IIIa antigen, which was expressed by the majority of the leukemic blasts in all three cases. Cases with evidence of multilineage differentiation of the leukemic transformation were excluded. A striking feature in two patients was the manifestation of lytic bone lesions and soft tissue masses at presentation. A biopsy of a lytic bone lesion and soft tissue mass in one patient revealed a megakaryoblastic leukemic infiltrate, which by immunocytochemical staining was positive for the megakaryocytic markers, glycoprotein IIb/IIIa antigen, and Factor VIII (von Willebrand factor) antigen. In contrast to granulocytic sarcomas, the megakaryoblastic sarcoma did not stain cytochemically for chloroacetate esterase. The mean survival after acute transformation was 5.3 months. The three cases of megakaryoblastic transformation represented a significant proportion of all CGL blastic transformation cases (ten cases) evaluated by bone marrow examination in our institution during a 13-month period. Megakaryoblastic transformation of CGL may occur more frequently than has been appreciated, and can present as lytic bone lesions or as soft tissue megakaryoblastic sarcomas.