Background: Telomere/telomerase interplay is involved in the regulation of genomic stability and cellular replicative potential. In CRC, shortening of telomeres promotes early steps of carcinogenesis but the prognostic role of telomere length is debated. High circulating levels of telomerase reverse transcriptase (TERT) appear to be a negative prognostic biomarker and polymorphisms in the telomerase RNA component (TERC) are associated with increased cancer risk. We hypothesized that genetic variants in the telomerase/telomere capping complex may predict first-line treatment outcome in mCRC pts. Methods: The impact on outcome of 21 selected SNPs within 13 genes (TERT, TERC, CLPTM1L, TERF1/2, POT1, ACD, NOP10, NHP2, GAR1, STN1, CTC1) was analyzed through the OncoArray, a custom array manufactured by Illumina, on genomic DNA from blood samples of 451 pts enrolled in two independent randomized trials. TRIBE FOLFIRI/bevacizumab (bev) arm (n=215, mPFS/OS: 9.7/26.2 mo) served as discovery cohort, FIRE-3 FOLFIRI/bev arm (n=107, mPFS/OS: 11.5/31.4 mo) as validation and FOLFIRI/cetuximab (cet) arm (n=129, mPFS/OS: 12.8/49.8 mo) as control. Results: In the discovery cohort, pts carrying any A allele of TERT rs2075786 showed longer mPFS (11.1 vs 9.3 mo, P= .021) and OS (33.5 vs 25 mo, P= .005) compared to the G/G genotype in multivariable analysis. Same results were observed for pts carrying any T allele of CLPTM1L rs401681 compared to the C/C genotype (P= .022). Any C allele carriers of TERC rs2293607 also showed longer mOS in multivariable analysis (P= .041) and the A/A genotype of ACD rs6979 was associated with longer mPFS in both uni- and multivariable analysis. Interestingly, the T/T genotype of CLPTM1L rs401681 was associated with shorter mPFS (7.8 vs 13.5 mo) in the cet cohort in uni- (P= .003) and multivariable analysis (P= .041). In subgroup analyses based on RAS mutational status several SNPs in additional genes were associated with PFS and/or OS in different cohorts. Conclusions: Our results suggest that SNPs in core telomerase/telomere capping complex genes may have a predictive and prognostic value in mCRC pts receiving targeted first-line treatments.