Objective To explore the efficacy and prognostic factors of cetuximab therapy for KRAS or all RAS wild-type(WT)metastatic colorectal cancer(mCRC).Methods We screened mCRC patients receiving at least two cycles of cetuximb and chemotherapy from those with KRAS WT(before November 2013)or all-RAS-WT(after November 2013)in the Department of Medical Oncology,Peking Union Medical College Hospital from November 2007 to December 2016. The relationship between the clinicopathological characteristics and the efficacy was retrospectively analyzed.Results A total of 60 patients were included. For the 34 patients receiving cetuximab as first-line treatment,the objective response rate(ORR)was 55.9%,and the progression-free survival and overall survival(OS)was 10 and 24 months,respectively. All-RAS-WT mCRC had significantly lower risk of progression than those with KRAS-only-WT(P=0.012),and left-sided colorectal cancer had higher ORR than right-sided colon cancer(62.1% vs. 0,P=0.033)during the first-line treatment. The median OS of the eight patients continuing cetuximab beyond first-line progression was 35.0(95%CI:23.6-46.4)months.Conclusions The efficacy of cetuximab for left-sided colorectal cancer was better than for right-sided colon cancer,and patients with all-RAS-WT have lower risk of progression than those with KRAS-only-WT. Patients benefiting from first-line cetuximab and continuing cetuximab beyond progression survive longer.