Mounting evidence suggests that activation of the mitogen-activated protein (MAP) kinase pathway plays an important role in tumorigenesis. MAP kinase/ERK kinase (MEK), a crucial constituent of this pathway, is activated by phosphorylation, and the phosphorylated MEK (pMEK) in turn activates ERK kinase. The expression of pMEK has been described in some human malignancies, but not in primary human colon tumors. In this study, we analyzed the expression of pMEK in 123 colorectal tumors by immunohistochemistry. pMEK was detected either in the cytoplasm (63 cases) or nucleus (40 cases) in 93 of the 123 tumors (76%). Tubular adenomas and villous adenomas also expressed pMEK in 30% and 40% of the tumors, respectively. By contrast, the epithelial cells in the normal colonic mucosa showed no or only weak expression of pMEK in the cytoplasm. Taken together, these results indicate that MEK is frequently phosphorylated in colorectal tumors, and suggest that phosphorylation of MEK may play a role in the development of colorectal tumors.