Background: Recently, the cancer stem cell (CSC) theory has been proposed, and CD133 has been suggested as a potential marker of CSCs in metastatic colorectal cancer (mCRC). Cetuximab, an IgG1 anti-EGFR chimeric mouse/human monoclonal antibody, has been approved for the treatment of mCRC. The purpose of this study is to evaluate the prognostic significance of CD133 expression in mCRC treated with cetuximab in combination with chemotherapy. Methods: We evaluated the prognosis of pts with mCRC treated with cetuximab retrospectively, and performed immunohistochemical staining to analyze the CD133 status. Non-parametric statistics, univariate and multivariate analysis were used. Results: From October 2008 to June 2009, 56 pts with measurable metastatic colorectal cancer were received cetuximab and 43 were evaluable. Patients characteristics were as follows : median age : 59.6 years (range 28-80) , PS 0/1 : 30/13 , colon/rectum : 28/15 , metastatic site (liver +/- : 35/8 , lung +/- : 32/11 , bone +/- : 6/37 , peritoneum +/- : 6/37 , lymph node +/- : 11/32 , local +/- : 3/40), best response rate was 9.3% (CR/PR/SD/PD : 1/3/19/11). Compared with CD133- pts with colorectal cancer , the progression-free survival (PFS) of CD133+ pts was significantly better (5.5 month; 95% CI, 4.4-6.7,p=0.026), and median overall survival (OS) was also significantly better (11.0 month; 95% CI, 5.4-16.5, p=0.002). In univariate analysis, liver metastasis, lung metastasis, peritoneal metastasis, lymph node metastasis, age, and CD133 at the baseline predicted PFS, and age, gender, liver metastasis, lung metastasis, bone metastasis, peritoneal metastasis, lymph node metastasis at the baseline, the presence of skin rash, and CD133 predicted OS. In order to evaluate the independent predictive effect of chemotherapy, multivariate Cox regression analysis was carried out. It showed that CD133 was the strongest predictor. Conclusions: CD133 status at the baseline was correlated with the prognosis of patients treated with cetuximab, suggesting that CD133 status might play a role to estimate the prognosis.