The expression of CD56 antigen is associated with poor prognosis in patients with acute myeloid leukemia certain neoplastic cells such as AML cells,(10,13-17) acute myeloid/NK cell leukemia (myeloid/NK AL),(17) ALL(10) and lymphoma.(11,18-21) The prognostic meaning of cell surface antigen expressions in AML is still a controversial matter.(7) CD56 expression is considered by some authors to have prognostic value in AML patients and is apparently associated with short overall survival,(14-16,22, 23) lower CR rates(14,22) and shorter duration of CR.(14,15, 23) Baer et al. demonstrated that CD56 expression in AML with t(8;21)(q22;q22), usually associated with a high CR rate and prolonged disease-free survival, was in fact significantly correlated with a short CR period and low OS.(15) Ferrara et al. reported that there was no association between CD56 expression and the rate of CR in patients with AML M3(23) differently from Murray et al. who found a lower CR rate in this subtype of CD56+ AML.(14) Both authors however observed shorter OS and duration of CR in AML M3 patients with CD56 expression. Raspadori et al.(22) found that CD56 expression was associated with a lower rate of CR. On the other hand, in a study including all LMA subtypes except M3, Chang et al.(13) demonstrated that the CR rate was not associated with CD56 expression, but with CD34 and HLA-DR expression. Some authors report that the presence of the CD56 marker is associated with a higher incidence of central nervous system (CNS) disease in patients with AML,(15,16) myeloid/ NK AL,(24) ALL(10) and lymphoma.(11) Moreover, EMI patients have demonstrated a lower CR rate and short OS.(16) Because CD56 is an NCAM isoform that plays an important function in neuronal growth and migration through cell to cell adhesion, there is a hypothesis that CD56 expression in leukemic cells could allow infiltration through the adhesion of these cells to tissues that also express this marker.(10,25) Ravandi et al.(10) demonstrated a high incidence of CNS disease in adult ALL patients who expressed CD56. While CD56 expression in MM had no prognostic value in one study,(26) it was associated with worse outcomes in another two studies.(27) NK cell or NK-like CD56+ lymphoma cells in extranodal sites such as the skin, which is not uncommon, have a bad prognostic value almost without exception.(28) In the present study, we investigated whether CD56 expression in AML is a risk factor for shorter OS and a higher death rate during induction chemotherapy in patients treated at Hospital de Clínicas of Porto Alegre (HCPA). Also, the incidence of CD56 expression in different AML FAB subtypes was addressed. Methods Patients This study involved a cohort of 48 AML patients admitted to HCPA who were diagnosed from July 2002 to July 2005. The group included 29 male and 19 female patients Rev Bras Hematol Hemoter. 2011;33(3):202-6 (proportion 1.4:1). The patients' ages varied from three to 78 years old with a median of 43 and a mean of 20.5 years old. The most frequent AML subtype observed was FAB M3 (25%) followed by FAB M1 (22.9%) and FAB M4 (14.6%). Data were obtained by consulting the medical records of AML patients treated by the hematology team of Hospital de Clínicas in Porto Alegre. The data of interest were: gender, age, FAB classification, leukocyte count at diagnosis, CR rate, disease-free survival and OS. All non-M3 AML patients received induction with Cytarabine 100 mg/m2/d as a continuous infusion, for seven days associated with Daunorubicine 45 mg/m2/day for three days ('7+3' scheme). The post-induction phase was carried out with one consolidation cycle (same drugs and doses as the induction phase) and two intensification cycles consisting of three days of Idarubicin (10 mg/m2/day) and high-dose Cytarabine (3 g/m2/day – three days). Patients with promyelocytic leukemia (M3) were treated with the AIDA protocol of the Italian group GIMEMA. Patients submitted to bone marrow transplant were not included in this study. Morphological analysis The cytomorphological analysis was performed using a bone marrow smear or a peripheral blood smear after anticoagulation with EDTA stained with May Grunwald- Giemsa. The bone marrow cytological exam subjectively assessed the cellularity, assessed hematopoiesis, and quantified megakaryocytes, blasts and the myeloid: erythroid ratio. Cytochemical analysis The PAS (periodic acid-Schiff) reaction and Sudan Black staining were used for the cytochemical analyses. PAS identifies cytoplasmic glycogen through the action of the periodic acid, which oxidizes protein carbohydrate radicals and converts them into aldehydes, which are stained red or a strong pink color by the Schiff's reagent; Sudan Black stains the lipid membrane of lysosomal granules in mature and immature myeloid cells of granulocytic and monocytic lineage due to the presence of intracellular phospholipids.(1) Immunophenotyping Immunophenotypic analysis of bone marrow blasts and of peripheral blood was performed according to a standard technique.(4) The sample was acquired using CellQuestT software in a FACSCalibur BD (Becton, Dickinson) flow cytometer, with five simultaneous reading parameters, three of which were fluorescence parameters (FL1, FL2, FL3) and then analyzed using Paint-a-Gate software in the mononuclear gate. The monoclonal antibodies marked directly with phycoerythrin (PE) fluorochromes or fluorescein isothiocyanate (FITC) (Becton Dickson, Immunotech, DAKO, 203