The unique aspects of acute promyelocytic leukemia. Acute promyelocytic leukemia (APL) accounts for approximately 10% of cases of acute myeloid leukemia (AML). Distinctive features of this disorder at the time of diagnosis include leukopenia coexisting with a marrow replaced with granulated dysplastic promyelocytes, disseminated intravascular coagulopathy, lack of Ia (HLA-DR) antigen expression, and translocation between the long arms of chromosomes 15 and 17 (t[15;17]). Heparin is widely but not universally used to interfere with the coagulopathy during the initial phases of treatment. Serial bone marrow examinations during the induction period demonstrate the achievement of remission despite the persistence of malignant-appearing promyelocytes. Patients with APL are generally younger than those with other subtypes of AML, have a 70% to 80% likelihood of entering remission, and are thought to have a more favorable prognosis than other individuals with AML. Remission may be achieved with a conventional anthracycline-cytarabine combination, anthracycline alone, or, apparently, all-trans retinoic acid. Genes potentially important in myeloid differentiation such as granulocyte colony-stimulating factor (G-CSF) and myeloperoxidase are located close to the breakpoint in the t(15;17) but have not been conclusively shown to be rearranged in the translocation. A better understanding of the unique aspects of APL may well shed light on the pathogenetic processes of AML. 