527 Background: The treatment benefits of epidermal growth factor receptor (EGFR) monoclonal antibodies for patients with KRAS mutations have not been demonstrated. However, some studies have suggested that all KRAS mutations are not equivalent, and that KRAS G13D mutations might have some survival benefit. Methods: We retrospectively analyzed the efficacy and toxicity of treatment with EGFR monoclonal antibody in 8 patients with KRAS G13D mutations and 5-FU/oxaliplatin/irinotecan (CPT) refractory metastatic colorectal cancer compared with 94 KRAS wild type patients at the National Cancer Center Hospital. Results: Eight patients with KRAS G13D mutations were treated with anti-EGFR monoclonal antibodies between July, 2009 and July, 2011. The median age was 66 (42-70); male/female 6/2; PS was 0/1/2, 2/5/1; treatment regimen was cetuximab/ cetuximab+CPT/ panitumumab+CPT, 2/5/1. Response rate (RR) was 12.5% and disease control rate (DCR) was 50.0% with 1 PR, 3 SD, and 4 PD. The PR case treated with cetuximab+CPT showed marked regression of tumor and long duration of treatment (9 months). The progression free survival (PFS) of 2 SD cases was 4.2 and 3.9 months. The other SD case is now on treatment. The median PFS of the 8 patients was 2.1 months (95% confidence interval [CI]: 0.0-5.2). The median overall survival (OS) has not been reached. Grade 3/4 toxicities included 1 hypomagnesemia G4 and 1 rash acneiform G3. Meanwhile, 94 KRAS wild type patients treated with anti EGFR monoclonal antibodies had an RR of 22.3% and DCR was 66.0% with 21 PR, 41 SD, 30 PD, and 2 NE. PFS was 5.6 months (95% CI: 4.9-6.3) and OS was 8.6 months (95% CI: 6.5-10.7). Conclusions: In this analysis, we identified one PR to anti-EGFR monoclonal antibody in a patient with KRAS G13D mutation and chemo-refractory metastatic colorectal cancer. However, we were unable to demonstrate equivalent efficacy in patients with KRAS G13D mutations and KRAS wild type patients. Further studies are needed to evaluate the efficacy and prognosis for this treatment.