Flow cytometric expression of bcl-2 protein was analyzed in 67 newly diagnosed acute myeloblastic leukemia (AML) patients using an anti-bcl-2 monoclonal antibody by direct immunofluorescence technique and results were correlated with FAB subtype, CD34 expression and clinical outcome. The number of bcl-2+ cells in each sample was heterogenous (range, 19% to 96%), with a mean of 81%. The percentage of bcl-2+ cells was higher in M0 and M1 types according to French-American-British classification. The mean fluorescence index (MFI), expressed as the ratio of sample channel:control mean channel was significantly higher (p=0.01) in M0 (19.0) and M1 (17.6) than M4 (11.7) and M5 (8.9) cytotypes. In addition, bcl-2 MFI significantly correlated both with CD34 positivity and with CD34 MFI. High percentage expression of bcl-2 and MFI index of bcl-2 was associated with a low complete remission rate after intensive chemotherapy (40.4% in cases with 20% and more positive cells vs 72% in cases with less than 20% positive cells). By statistical analysis we also demonstrated that both bcl-2 high MFI (>16) and CD34 expression are independent prognostic factors for achieving CR in AML.