Objective: The aim of this study was to investigate the relationship between expression of MEK1 protein in the mitogen-activated protein kinase (MAPK) signaling pathway and liver as well as lymph node metastasis in colorectal cancer patients.Methods: Immunohistochemistry was performed to detect the expression of MEK1 protein in primary cancer, normal colonic mucosa, lymph nodes and liver metastatic foci from 86 colorectal cancer patients.Life table analysis was employed to evaluate the association between MEK1 expression and patients survival.Results: The positive rate of MEK1 expression in the primary cancer, normal colonic mucosa, metastatic lymph nodes and liver metastatic foci was 52.3%, 32.6%, 71.4% and 78.3%, respectively.The positive rate of MEK1 expression in the primary cancer, metastatic lymph nodes and liver metastatic foci was significantly higher than that in the normal colonic mucosa (P < 0.01).Furthermore, the positive rate of MEK1 expression in stage III and IV colorectal cancer patients was dramatically higher than that in stage I and II colorectal cancer patients (P < 0.01).The positive rate of MEK1 expression in patients with poorly differentiated adenocarcinoma and mucinous adenocarcinoma was significantly higher than patients with well or moderately differentiated adenocarcinoma (P < 0.01).The 3-year disease-free survival rate was 41.3% in MEK1 positive patients and 73.1% in MEK1 negative patients.The survival rate of MEK1 positive patients was significantly lower than that of MEK1 negative patients (P < 0.05).Conclusion: The increased expression of MEK1 was associated with lymph node metastasis and liver metastasis of colorectal cancer.Therefore, detection of MEK1 expression may have important significance in the evaluation of patients prognosis.