Background: Sub-analyses of US and European randomized trials have demonstrated that primary tumor location is a critical prognostic factor in mCRC treated with 1st-line chemotherapy; moreover, left-sided tumor location may be a predictor of cetuximab efficacy in KRAS exon 2 wild-type tumors (Loupakis F, et al. J Natl Cancer Inst 2015; von Einem JC, et al. J Cancer Res Clin Oncol 2014). We therefore investigated the prognostic impact of primary tumor location on outcomes of Japanese pts enrolled in JACCRO CC-05 or CC-06 trial, which evaluated efficacy of cetuximab in combination with FOLFOX or SOX, respectively, for mCRC with KRAS exon 2 wild-type tumors. Methods: This study evaluated the association of tumor location with overall survival (OS) and progression-free survival (PFS) in mCRC pts from 2 phase II trials of 1st-line therapy; JACCRO CC-05 of cetuximab plus FOLFOX (n= 57, UMIN000004197) and CC-06 of cetuximab plus SOX (n= 67, UMIN000007022). Tumors proximal or from left flexure to rectum were defined as right-sided or left-sided, respectively. Results: In total of 124 pts of the 2 trials, 110 pts were assessable for the primary tumor location: 90 pts with left-sided tumors and 20 pts with right-sided tumors. In the population consists of 110 evaluable pts, median PFS was 9.4 months, and median OS was 33.9 months. Left-sided tumors were significantly associated with longer OS (36.2 months vs. 12.6 months, HR 0.28, 95%CI 0.15-0.53, p< 0.0001) and PFS (11.1 months vs. 5.6 months, HR 0.47, 95%CI 0.29-0.82, p= 0.0041) compared to right-sided tumors. The association was evident in the group of FOLFOX (p< 0.0001 for OS and p= 0.0002 for PFS), while there was a trend in OS of the group of SOX (p= 0.079). In the FOLFOX-group, median OS and PFS were 5.7 and 3.0 months, respectively, for right-sided tumors (n= 9) and 42.8 and 11.3 months, respectively, for left-sided tumors (n= 43). Conclusions: Our study demonstrates that primary tumor location may serve as a predictor of prognosis of mCRC pts treated with cetuximab plus oxaliplatin-based therapy, potentially confirming the prognostic impact of tumor location.