Background Circulating cell-free DNA (cfDNA) in plasma is a mixture of DNA from malignant and normal cells, and can be used as a liquid biopsy to detect and quantify tumour specific mutations e.g. KRAS. We investigated the clinical value of KRAS mutations when detected in plasma compared to tumour in patients from metastatic colorectal cancer (mCRC) prior to anti-epidermal growth factor receptor (anti-EGFR) therapy. Secondly, we investigated the concentration of total cfDNA in relation to clinical outcome. Patients and methods Patients were resistant to 5-FU, oxaliplatin and irinotecan and treated with 3rd line irinotecan (180 mg/m2) and cetuximab (500 mg/m2) q2w in a prospective phase II trial. The study was conducted prior to implementation of KRAS as selection criteria. Plasma was obtained from a pre-treatment EDTA blood-sample, and the total cfDNA, and KRAS mutations were quantified by an in-house qPCR method. Results are presented according to REMARK. Results One-hundred-and-forty patients were included. Thirty-four percent had detectable KRAS mutations in the tumour, compared to 23% in plasma. KRAS detection in archival tumour tissue showed no correlation to survival, whereas plasma KRAS status remained a strong predictive and prognostic factor in multivariate analysis (Hazard Ratio (HR) = 2.98 (95% CI 1.53-5.80, p = 0.001) and 2.84 (1.46-5.53, p = 0.002), for OS and PFS, respectively). Combining the information of total cell free DNA levels and plasma KRAS mutation status, produced an additional prognostic effect. Conclusion The value of clinically relevant mutations could be improved by performing the analysis on circulation plasma DNA rather than archival tumour tissue.