OBJECTIVE To investigate the expressions of cell surface differentiation antigen CD56 and CD11b antigen in acute monocytic leukemic (AML-M(5)) cells and their clinical significance. METHODS A total of 113 cases of de nove adult AML-M(5) were examined genetically and immunologically using G-banding technique, interphase fluorescence in situ hybridization (I-FISH) and flow cytometry immunophenotyping, and the results were analyzed in relation to their clinical data. RESULTS Of the 113 cases, the expression rates of CD56 and CD11b was 28.32% and 73.45%, respectively. The CD56(+) patients had high CD11b expression, and the expression levels of CD11b and CD56 were positively correlated (P<0.05). The incidence of karyotypic abnormalities was 48.57% (55 cases) in these patients, including 25 (22.12%) with 11q23 aberrations. Twenty-five cases were positive for MLL gene abnormalities as found by I-FISH analysis. Compared with the patients positive for both CD56 and CD11b, those negative for both CD56 and CD11b showed increased peripheral blood white blood cell (WBC) count and also increased blast and progenitor cells in the bone marrow (P<0.05); the former patients often had karyotypic abnormalities, commonly involving 11q23 aberrations (P<0.05), whereas the latter patients presented more likely with extramedullary infiltration and refractory leukemia (P<0.01) with lowered complete remission rate and shortened median survival time (P<0.01). CD56-positive patients were more likely to have karyotypic abnormalities and refractory leukemia than CD11b-postive patients (P<0.05), but the peripheral blood WBC counts, bone marrow blast and progenitor cells, extramedullary infiltration, complete remission rate or median survival time showed no significant differences between them (P>0.05). CONCLUSION AML-M(5) patients with CD56 positivity and high expression of CD11b often have aberrant karyotypes, commonly involving 11q23/MLL gene abnormality. These patients frequently develop extramedullary infiltration and refractory leukemia often with poor prognosis.