Background: The introduction of immunotherapy has significantly improved outcome in various tumors. Immune stimulating proteins exert an anti-tumor effect mainly through enhancing T-cell mediated immune response. Additionally, preliminary data suggest a major role of immune stimulating proteins in modulating angiogenesis. We therefore hypothesize that variations in genes involved in the immune activation pathway may predict outcome in pts with mCRC treated with first-line FOLFIRI/ bevacizumab (bev). Methods: The impact of 4 functional SNPs within the CD40L, Light, OX40L and ICOS genes on outcome was evaluated in 322 pts with mCRC treated with first-line FOLFIRI/bev in two randomized phase III trials. We used TRIBE as a discovery (n = 215) and FIRE-3 as a validation set (n = 107). One hundred twenty-nine pts treated with FOLFIRI/cetuximab (cet) served as a control cohort (FIRE-3). OncoArray, a custom array manufactured by Illumina was used for data extraction. Genomic DNA was extracted from blood. Results: Baseline characteristics: FOLFIRI/bev, discovery set (TRIBE), median PFS/OS/FU 9.7/26.2/48.9 mo; FOLFIRI/bev, validation set (FIRE-3), PFS/OS/FU 11.5/32.4/71.1 mo; FOLFIRI/cet, control set (FIRE-3) PFS/OS/FU 12.8//23.9/70.7 mo. The CD40L rs1126535 SNP showed significant association with OS. Pts in the discovery cohort harboring any T allele and treated with FOLFIRI/bev had a longer median OS compared to C/C carriers (27.9 vs. 20.0 mo) in both univariate (HR 1.83, 95% CI 1.19-2.81, p = 0.005) and multivariate analyses (HR 1.62, 95% CI 1.03-2.56, p = 0.038). Similarly, any T allele carriers in the validation cohort had a significantly longer median OS than those harboring a C/C genotype (40 vs. 19.0 mo) in the multivariate analysis (HR 2.80, 95% CI 1.05-7.50, p = 0.040). However, this association could not be shown in pts receiving FOLFIRI/cet (HR 0.60, 95% CI, 0.18-1.94, p = 0.38). Conclusions: We show for the first time that the CD40L polymorphism rs1126535 might serve as a predictive marker in pts with mCRC treated with FOLFIRI/bev. Targeting CD40L might be promising to further improve treatment against mCRC and to overcome resistance to anti-angiogenic therapy.