681 Background: Cetuximab (Cmab) + chemotherapy is now indicated in various treatment guidelines as a first-line therapy for non-resectable metastatic colorectal cancer (mCRC). The efficacy of Cmab + FOLFIRI has been demonstrated in CRYSTAL, but its efficacy and safety in combination with other regimens is unclear. There are few reports on Cmab combination therapy as first-line chemotherapy in Japan, and there are no reports of treatment results in the ESMO guideline groups 1, 2, and 3. Therefore an observational study was planned to evaluate the efficacy and safety of first-line therapy including Cmab, and to clarify the treatment results by group. This study received support from the Public Health Research Foundation’s Comprehensive Support Project for Oncology Research (CSPOR). Methods: The subjects were mCRC patients who received Cmab as first-line therapy either as combination therapy or monotherapy. Endpoints were response rate, PFS, and reason for discontinuation of first-line treatment (curative resection, PD, adverse events). To evaluate safety, skin conditions (incidence, degree, previous history) and other adverse events were investigated. Results: Among 579 patients enrolled at 158 institutes from January 2012 until June 2013, 575 were subjected to analysis. Background factors were PS 0/1/2 at 406/143/26, males females at 368/207, median age of 65 (31-88), groups 1/2/3 at 172/229/174, and concomitant drugs were oxaliplatin/irinotecan/others at 377/173/25. Response rate in the evaluable cases was 57%, and in groups 1/2/3 was 65%/54%/53%. The 1-year survival rate in groups 1/2/3 was 93%/61%/78%. In adverse events, the incidence of skin conditions (rash acneform) at 8 weeks/16 weeks was 69%/60% in patients with no history of acne vulgaris (2%/8% at G3 or higher) and 83%/81% in those with a previous history (13%/14% at G3 or higher). Conclusions: We reported treatment results according to ESMO guideline groups on the efficacy and safety of Cmab as first-line therapy in mCRC patients. Response rate and survival were higher in group 1. Survival outcomes after 1 year will be followed up. Clinical trial information: UMIN000007275.