To investigate the clinical role of the soluble form of c-kit receptor (s-kit) in patients with acute myeloid leukemia (AML), we determined the levels of serum s-kit and expression of c-kit antigens and mRNA in leukemic cells. The serum s-kit level was measured using ELISA assay in 30 AML patients and 20 normal controls. C-kit antigens of leukemic blasts were stained immunohistologically, and c-kit mRNA was detected by RT-PCR. The serum s-kit level in M1 and M2 were significantly increased (p<0.01) and that in M4 or M5 was significantly decreased (p<0.05) compared to that in the controls. In the comparisons among subtypes of FAB classification, M1 and M2 showed significantly higher levels than M4 or M5 (p<0.05 and p<0.01, respectively). Both expression of c-kit antigens and mRNA were observed in M0 (1/4), M1 (2/4) and M2 (6/8), but neither was observed in M4 or M5. The serum s-kit levels were correlated with the absolute number of AML blasts in peripheral blood (r=0.564, p<0.05). These results indicate that the serum s-kit level is related to the stage of differentiation of AML blasts in accordance with the expression of c-kit protein and mRNA in AML blasts, and is useful for assessment of leukemic cell burden.