Background: Associations of EGFR gene copy number (GCN) and EGFR CA-SSR1 polymorphism with clinical outcome still remain controversial in mCRC pts treated with cet. Interethnic differences in the repeat number of EGFRCA-SSR1 have been reported between Caucasian and Asians, however, there have been few reports about the associations with clinical outcome in Japanese pts. The aim of this study was to evaluate the predictive value of two biomarkers in Japanese mCRC pts treated with cet. Methods: This study enrolled 77 pts with tumor available from two prospective clinical trials evaluating combination of cet with oxaliplatin-based chemotherapy as first-line treatment in mCRC pts with KRAS wt and EGFR-expressing tumors, modified FOLFOX6 (n=28/57, UMIN000004197) and SOX (n=49/67, UMIN000007022). Genomic DNA isolated by macro-dissection from tissue was screened for the EGFR GCN determined by FISH on FFPE tumor specimens with the cut-off value of 2.9, adopted as previously reported (Cappuzzo F, et al., 2008), and the EGFR CA-SSR1 short (S; ≤19) / long (L; ≥20) determined by PCR amplification and fragment length analysis using capillary electrophoresis. Associations of the biomarkers with efficacy, including response, progression-free survival, and overall survival, were evaluated. Additional analysis was addressed at a possible association between the EGFRCA-SSR1 and ST. Results: The frequency of the L alleles of the EGFR CA-SSR1 was 64%. There was no significant association between two biomarkers and efficacy. However, the EGFR CA-SSR1 variant significantly correlated with ST evaluated on 8 weeks after initial administration. The rate of ST with grade 2 or 3 was 33% (10/30), 19% (5/26), and 64% (7/11) in LL, SL, and SS genotype, respectively (fisher's exact test p=0.03). Conclusions: Our study provides first evidence that the EGFR CA-SSR1 variant was associated with cet-induced ST in Japanese mCRC pts. No predictive value for cet treatment could be identified in our screened biomarkers, however, extended RAS mutations analysis is warranted (UMIN000010635).