Background: The prognostic value of BRAF and KRAS mutations in resected CC pts remains controversial due to published studies that include stage II & III, microsatellite instability (MSI) and MSS, colon and rectal tumors, and variable treatment regimens. We examined this question in prospectively collected biospecimens from MSS stage III CC pts receiving adjuvant FOLFOX +/- cetuximab. Methods: Tumors were analyzed for BRAF V600E and KRAS exon 2 mutations, only MSS tumors were included. Three groups were defined: BRAF Mutant, KRAS Mutant and double wild-type (WT). The analytic strategy estimated study- and arm-specific prognostic effects to assess homogeneity of results, and then analysis of pooled data. Associations of mutations with time-to-recurrence (TTR), survival after relapse (SAR) and overall survival (OS) were analysed using a stratified Cox proportional hazards model. Multivariate models were adjusted for treatment and covariates (age, sex, tumor grade, T/N stage, tumor location, ECOG PS). Results: Of the 5,577 pts enrolled, 3,934 tumors were MSS and evaluable for BRAF and KRAS; 279 pts were BRAF Mutant, 1,450 KRAS Mutant, and 2,205 WT. Both mutations were linked to shorter TTR and OS vs WT, and results were confirmed in multivariate analyses (table). Median SAR was 2.57, 2.09 and 1.0 year in WT, KRAS Mutant (HR: 1.20- 95%CI: 1.01-1.44, p < 0.0001) and BRAF mutant (HR: 3.01-95%CI: 2.32-3.93, p < 0.0001), respectively. No interaction was found between treatment (with or without cetuximab) and KRAS/BRAFmutations for TTR (p = 0.38) or OS (p = 0.16). Conclusions: In a large pooled analysis of pts with resected stage III MSS colon cancers receiving adjuvant FOLFOX, BRAFV600E or KRAS exon 2 mutations, including codons 12 or 13, are independent predictors of significantly shorter TTR, SAR and OS. Future clinical trials in the adjuvant setting should consider these mutations as important stratification factors.