Acute monocytic leukemia is a distinct subtype of acute myeloid leukemia (AML) with characteristic biology and clinical features. This study was designed to compare the immunophenotypical features and clinical manifestations of the patients with AML-M(5a) to that of patients with AML-M(5b), and to identify differences between M(5a) and M(5b) and to explore their relations. A total of 58 cases of de novo adult patients with AML M(5) were investigated. Immunofluorescence analysis by flow cytometry was performed to determine the immunophenotype of the leukemic cells in all cases. Meanwhile, clinical data of these cases were studied retrospectively. The results showed that the immunophenotypes of monocytic leukemic cells in patients with AML M(5) were heterogeneous, and CD68 and CD11b were expressed higher in patients with AML M(5a), compared with that in patients with AML M(5b) (P < 0.01). The significant differences in sex, extramedullary infiltration, WBC counts of peripheral blood, complete remission rate and disease-free survival (DFS > 300 days) between the patients with AML M(5a) and M(5b) did not exist (P > 0.05). It is concluded that the special individual immunophenotype features can be detected in patients with either of AML M(5a) or M(5b), and that expressions of CD68 and CD11b were much higher in M(5a). It seems that the complete remission rate and disease-free survival of patients with M(5a) and M(5b) are not different from that of currently available therapy.