Abstract: OBJECTIVE To determine the impact of unrecognized renal insufficiency(URI) on safety and efficacy of chemotherapy in metastatic colorectal cancer (mCRC). METHODS Medical records of mCRC patients with normal SCr started on capecitabine plus oxaliplatin(XELOX) were retrospectively reviewed. CrCL was estimated using the Cockcroft-Gault formula. The patients were divided into URI group and control group, and the toxicities and effectiveness of therapy were compared between the two groups. RESULTS A total of 143 patients were enrolled, in which 34.9% experienced URI. The URI group developed significantly more grade 1-2 toxicities including myelosuppression, diarrhea, stomatitis and hand-foot syndrome. The response rate and TTP were significantly lower in URI group, whereas the OS was similar in two groups. CONCLUSION URI can increase the toxicity and decrease the survival of XELOX-treated mCRC patients. Renal function screening via CrCL estimation is required for all mCRC patients before initial chemotherapy.