Background: The clock machinery comprises a complex network of transcription-translation feedback loops which regulates the circadian expression of target genes involved in key cellular functions. The disruption of the circadian clock has been associated with increased CRC risk, and the expression levels of core clock genes with clinicopathological features, survival and treatment response. We therefore hypothesized that genetic variants in clock genes may predict first-line treatment outcome in mCRC pts. Methods: Genomic DNA from blood samples of pts enrolled in two independent randomized phase III trials, TRIBE and FIRE-3, was genotyped through the OncoArray, a custom array manufactured by Illumina including approximately 530K SNP markers. The impact on outcome of 22 selected SNPs in 11 genes of the clock pathway (CLOCK, BMAL1, NPAS2, PER 1-2-3, CRY 1-2, RORa, hTIM, SIRT1) was analyzed. Results: A total of 451 pts were included in the analysis. TRIBE FOLFIRI/bevacizumab (bev) arm served as discovery cohort (n = 215, median PFS/OS: 9.7/26.2 mo), FIRE-3 FOLFIRI/bev arm as validation (n = 107, mPFS/OS: 11.5/31.4 mo) and FOLFIRI/cetuximab arm as control (n = 129, mPFS/OS: 12.8/49.8 mo). In the discovery cohort, the overall population carrying the 3’UTR CLOCKrs3749474 T/T variant showed a shorter mPFS (8.8 vs 10.4 mo) compared to pts with any C allele both in univariate (HR = 1.69; 95%CI 1.11-2.59; p = 0.012) and in multivariable analysis (HR = 2.06; 95%CI 1.26-3.37; p = 0.004). This effect was stronger in female pts (p < 0.001), and left-sided CRCs (p = 0.002). Significant interaction was found with gender (p = 0.005); however, no interaction was found with tumor location or RAS status. Findings were validated in overall pts in FIRE-3 bev cohort both in univariate (HR = 1.99; p = 0.027) and in multivariable analysis (HR = 2.45; p = 0.013). No significant association was observed in the control arm. Conclusions: Our results provide the first evidence that CLOCKrs3749474 polymorphism may have a predictive value in mCRC pts treated with first-line FOLFIRI/bev. This finding supports a possible role of clock genes in contributing to resistance to anti-VEGF treatment.