Differentiation inhibitory factor (nm23 protein) inhibited the induction of differentiation of mouse myeloid leukemia M1 and WEHI-3BD+ and human erythroleukemia HEL, KU812, and K562 cells. Block of differentiation may be associated with the aggressive behavior of leukemia. To examine the role of nm23 in human myeloid leukemia, we investigated the relative levels of nm23-H1, nm23-H2, and c-myc transcripts in 42 patients with acute myelogenous leukemia (AML), and in 5 with chronic myelogenous leukemia at chronic phase by reverse transcriptase polymerase chain reaction. The expression of nm23-H1 and -H2 but not of c-myc in AML was significantly higher than that in normal blood cells. Among AMLs, acute monocytic leukemia (presentation with AML-M5 morphology) was especially associated with elevated nm23-H1 and -H2 mRNA levels. On the other hand, the elevated levels of c-myc expression in AML-M5 were less evident. An analysis of correlation between nm23 expression and clinicopathological parameters showed that resistance to initial chemotherapy is associated with increased nm23-H1 mRNA levels and that a high initial white blood cell count is associated with increased nm23-H2 mRNA levels. Elevated nm23-H1 mRNA levels were associated with significantly reduced the overall survival of AML, especially of AML-M5 patients. The present results indicate that nm23-H1 and -H2 are overexpressed in AML and especially nm23-H1 gene expression predicts the prognosis of AML, especially of AML-M5.