Background In spite of demonstrating prognostic and possibly predictive benefit in retrospective cohorts and meta-analyses of cancer populations, including colorectal cancer (CRC), prospective evaluation of the relationship between neutrophil to lymphocyte ratio (NLR) and treatment outcomes in previously untreated mCRC patients receiving bevacizumab-based therapy has not yet been performed. Methods An open-label, single arm, multi-centre study. Patients received first-line bevacizumab plus XELOX or mFOLFOX6 (Phase-A) and continued bevacizumab plus FOLFIRI beyond first progression (Phase-B). Analyses included the association of NLR with phase A progression free survival (PFS) and overall survival (OS). A sub-study investigated the safety in patients with the primary in situ tumor. An exploratory sub-study examined relationships of circulating proteomic markers with PFS. Results Phase-A enrolled 128 patients; median age was 64 years (range: 26-84), 70 (55%) were female, 71 (56%) were PS-0 and 51 (40%) had primary in situ tumor. Fifty-three (41%) patients entered Phase-B. The median baseline (b) NLR was 3.2 (range: 1.5-20.4) with 32 (25%) patients having bNLR > 5. The PFS hazard ratio (HR) by bNLR > 5 versus ≤ 5 was 1.4 (95% CI: 0.9-2.2; p = 0.101). The median PFS was 9.2 months (95% CI: 7.9-10.8) for Phase-A and 6.7 months (95% CI: 3.0-8.2) for Phase-B. The HR for OS based on bNLR > 5 versus ≤ 5 was 1.6 (95% CI: 1.0-2.7; p = 0.052). The median OS was 25 months (95% CI: 19.2-29.7) for the full analysis set and 14.9 months for Phase-B. Baseline levels of nine proteomic markers showed a relationship with PFS. Treatment related toxicities were consistent with what has previously been published. There were 4 (3%) instances of GI perforation, of which, 3 (6%) occurred in the primary in situ tumor group. Conclusions Results from this study are aligned with the previously reported trend towards worse PFS and OS in patients with higher bNLR.