AbstractAcute promyelocytic leukemia (APL) is characterized by proliferation of morphologically abnormal promyelocytes and a severe bleeding diathesis. The abnormal promyelocyte is characterized by abundant, large granules, many of which are spindle‐shaped. Electron microscopic appearance of the granules closely resembles that of Auer rods. The granules appear to possess tissue thromboplastin activity by both immunologic and clotting assays. Coagulation studies in APL are generally consistent with disseminated intravascular coagulation. Prolongation of the prothrombin time and elevation of fibrinogen degradation products are the tests that are most commonly abnormal. Although occasional reports indicate a favorable response of the coagulopathy to drugs that inhibit fibrinolysis, the use of prophylactic heparin appears to be the treatment of choice. The response rate of APL to chemotherapy regimens that contain an anthracycline is comparable to that of acute myelogenous leukemia. The recent description of the 15;17 chromosomal translocation which may be pathognomonic for APL is only the second example of a chromosomal marker of human neoplasia. Marked elevation of serum vitamin B12 and B12 binding proteins appears to be another characteristic feature of APL. An in vitro cell line of APL cells has been demonstrated to have the capacity to differentiate to functional polymorphonuclear leukocytes, but the cause for the maturation arrest is unknown.