Background: Cancer-associated inflammation has been identified as a key determinant of disease progression and survival in colorectal cancer. We investigated the association between circulating inflammatory cytokines and survival in metastatic colorectal cancer (mCRC) patients. Methods: Plasma levels of 47 cytokines were measured using multiplex-bead assays in a cohort of 168 previously untreated mCRC patients. Demographic, clinical-pathological features, body mass index, and mortality data were abstracted from patient medical records. Overall survival (OS) was evaluated by Kaplan-Meier analysis and Cox proportional hazards regression. Results: Using principal component analysis, we identified a subset of cytokines explaining the maximum variance in OS; and found interleukin (IL)-1b, IL-5, IL-8, IL-12 and VEGF to be significantly associated with OS. However, only IL-8 was significantly and independently associated with OS in multivariable-adjusted models. For each 100 pg/ml increase in the level of circulating IL-8, hazard rate for death increased by 1.6 (95% CI 1.24-1.97). IL-8 measurements ranged from <1 to 413 pg/ml with a median value of 22 pg/ml. Median uncensored survival was 26.5 and 15.5 months among patients with IL-8 levels below and above this value, respectively. ROC analysis of IL-8 demonstrated an AUC of 0.69 (95% CI 0.60-0.76), as compared to 0.52 for CEA (95% CI 0.46-0.59). Conclusions: We identified an association between IL-8 and OS in previously untreated mCRC patients, suggesting its potential role as a prognostic inflammatory biomarker. In this dataset, IL-8 outperformed CEA as a prognostic biomarker, a finding which requires validation in subsequent work. Appropriately identifying, monitoring and managing chronic inflammation and the host inflammatory response during colorectal cancer treatment may be important for improving long-term survival.