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Acute promyelocytic leukemia (APL) is not a morphologically homogeneous entity: to verify whether there is any relationship between this heterogeneity and other biological and clinical aspects, we studied 43 cases of APL with morphological, cytochemical, cytogenetic and immunological methods.Three morphological categories were present: a classic hypergranular type (30 cases), a microgranular type (6 cases) and a form with basophilic granules (M3b) that stained metachromatically with toluidine blue (7 cases). In all these groups there were cases with cytochemical features of both myeloid and monocytic type (alpha-naphthyl-acetate esterase positive). No immunological and cytogenetic differences were observed; the morphological variant with basophilic granules was more frequent in females; age distribution was not related to the morphological subtype; organomegaly was extremely rare in M3b. A low white blood cell count was constant in M3b, whereas no differences were observed in hemoglobin and platelet values. Severity of bleeding was worst in the group with toluidine blue metachromasia; this and the microgranular type had poor prognosis.Our study confirms the importance of identifying different cytologic categories in APL. In particular we focused our attention on a new variant with basophilic granules.