Background: Systemic inflammation, evaluated with neutrophil-to-lymphocyte ratio (NLR), and sarcopenia, assessed with skeletal muscle index (SMI), appear to predict mortality in many cancers, although the optimal cut-offs are still a matter of debate. The goal of this study is to evaluate the cut-offs of NLR and SMI and their prognostic value on metastatic colorectal cancer (mCRC) under first line treatment. Methods: Retrospective cohort analysis of patients with mCRC under first-line treatment in two hospital units, between January 2015 and December 2018. Muscle mass area (MMA) was detected on computed tomography, on L3 segment, with ImageJ software. SMI was determined as MMA/height². Statistical analysis was performed with IBM SPSS v25 software.Results: 178 patients were analysed, with mean age of 62 (±11) years old. 65% were male and 62.9% had ECOG PS 0. Two thirds of patients presented metastasis in a single organ, mainly in liver. The cut-off values were established based on ROC analysis. Systemic inflammation was defined as NLR > 2.80 and sarcopenia was defined by SMI < 49.12 cm²/m² for men and <35.85 cm²/m² for women. 52% of patients presented sarcopenia. In a multivariate analysis using a Cox regression model, an association was observed between overall survival and ECOG PS (p=0.014; HR 5.46, CI 95% [1.42-21.10]), systemic inflammation (p=0,038; HR 2.20, CI 95% [1,05-4.62]) and sarcopenia (p=0.01; HR 4.73, CI 95% [1.85-12.09]). Conclusions: In this study, sarcopenia and systemic inflammation showed independent prognostic value on patients with mCRC. Future prospective studies are necessary to establish a definitive cut-off value for application in clinical practice.