High levels of indoleamine 2,3-dioxygenase (IDO) are involved in tumour escape mechanisms. The aim of this study is the evaluation of L-kynurenine of plasma as marker of diagnostic and prognostic in patients with colorectal cancer. The study included 78 patients with colorectal cancer, of whom 15 % were in stage I/II, 30 % in stage III, and 55 % in stage IV, and was compared with a control group of 70 healthy subjects. The receiver operating characteristic (ROC) curve analysis showed an area under the curve of 0.917, with a specificity of 100 % and with a sensitivity to detect cancer of the colon of 85.2 %, taking 1.83 μM as a cut-off point. The overall survival analysis also indicated that patients with low levels of L-kynurenine in plasma increased survival rate after 45 months of follow-up (P = 0.032). These results show that the plasma levels of L-kynurenine could be a good biomarker to differentiate individuals with colorectal cancer from healthy individuals.