Objectives: The aim was to explore the prognostic significance of IL-6 and markers of systemic inflammatory response (SIR), in particular C-reactive protein (CRP), in metastatic colorectal cancer (mCRC) patients, in the total study population and according to RAS and BRAF mutation status. Results: High levels of pretreatment serum IL-6 or CRP were associated with impaired outcome, in terms of reduced PFS and OS. Patients with low versus high serum IL-6 levels had median OS of 26.0 versus 16.6 months, respectively (P < 0.001). Stratified according to increasing CRP levels, median OS varied from 24.3 months to 12.3 months, (P < 0.001). IL-6 and CRP levels affected overall prognosis also in adjusted analyses. The effect of IL-6 was particularly pronounced in patients with BRAF mutation (interaction P = 0.004). Materials and Methods: IL-6 and CRP were determined in pre-treatment serum samples from 393 patients included in the NORDIC-VII trial, in which patients with mCRC received first line treatment. The effect of serum IL-6 and CRP on progressionfree survival (PFS) and overall survival (OS) was estimated. Conclusions: High baseline serum consentrations of IL-6 or CRP were associated with impaired prognosis in mCRC. IL-6 and CRP give independent prognostic information in addition to RAS and BRAF mutation status.