Background: The combination therapy of Cetuximab (Cmab) + chemotherapy is one option as the first-line chemotherapy for KRAS wild-type metastatic colorectal cancer (mCRC). The efficacy of Cmab+FOLFIRI has been shown by CRYSTAL trail. There has been little experience of Cmab as the first-line chemotherapy in Japan, the data on Cmab combined with other drugs are insufficient, and efficacy according to the ESMO guideline groups (groups 1, 2, 3) has not yet been reported. Methods: The subjects were mCRC patients with PS 0-2 who received the first-line chemotherapy including Cmab, either in combination with chemotherapy or as monotherapy. Treatment duration, reason of discontinuation of the first-line chemotherapy (PD, conversion to surgery, adverse events), the second-line treatment, time to treatment failure, progression-free survival, response rate (RR), changes in paraneoplastic symptoms, skin symptoms (incidence, degree, prevention), and incidence and degree of other adverse events were investigated. Results: The 578 patients from the 158 institutes between January 2012 and June 2013 were enrolled. There were PS0/1/2 at 409/144/25, male/female at 368/210, median age of 65 (31-88) and groups 1/2/3 at 172/231/175. KRAS was wild type/mutant type/undetermined in 95.5%/2.4%/2.1%. Chemotherapies combined with Cmab were oxaliplatin-based/irinotecan-based/others (377/176/25). In evaluable cases, RR in groups 1/2/3 were 68.3%/50.8%/41.7%. In the frequency of skin symptoms reported up to 8 weeks/16 weeks, acneform rash was observed in 70.1%/62.1% (all grades) and in 1.2%/5.7% (grade 3 or higher), while paronychia was in 32.1%/40.6% (all grades) and in 2.6%/9.4% (grade 3 or higher). Liver resection rate and survivals are to be examined in the future. Conclusions: This is the first report on the first-line chemotherapy using Cmab, with RR as efficacy and skin symptoms as safety. Efficacy of the patients with the first-line chemotherapy using Cmab is different among 3 groups. This study is sponsored by the Public Health Research Center Foundation CSPOR in Japan. Clinical trial information: UMIN000007275.