Immunological phenotyping of acute leukemia have provided enormous and important information for the classification and lineage determination of leukemia. Forty-nine patients with CD7(+) CD56(+) acute myeloid leukemia (AML) were analyzed. There were 17 patients of M0, which corresponded to myeloid/NK cell precursor acute leukemia, and 32 patients of AML other than M0 (9 each for M1 and M2, one for M3, 3 for M4, 4 for M5 and 6 for M7). Age distribution was similar between these two groups, but CD7(+) CD56(+) M0 showed significant male predominance than CD7(+) CD56(+) M1-M7 (M:F = 15:2 vs. 15:17, P = 0.006). The disease localization and the hematological manifestations were different, showing fewer white blood cell counts and circulating leukemic blasts, less anemia, less thrombocytopenia and more frequent extramedullary involvement in M0 group. The prognosis was poor in both groups, and there was no statistical difference. These findings suggest that extramedullary involvement of myeloid/NK cell precursor acute leukemia is not directly derived from the presence of CD7 and CD56 antigens on leukemic cells. The poor prognosis of CD7(+) CD56(+) M1-M7 suggests that this phenotype may act as a prognostic factor for AML, but this should be confirmed in further studies.