Our aim was to investigate the effect of chemotherapy on plasma total antioxidant capacity and polyphenols in patients with colon cancer. Plasma samples were collected from 70 CRC patients under chemotherapy treatment, and 15 non-treated patients. The control group included 71 healthy individuals. Plasma ABTS and FRAP were measured as biomarkers of antioxidant total capacity and the total phenols as an indicator to determine the polyphenols levels in plasma. Treatment with chemotherapy protocols resulted in a significant decrease of ABTS (-24 %, p < 0.048), FRAP (-15 %, p < 0.046) and polyphenols (-46 %, p < 0.05) compared with the values of those not treated. The total antioxidant capacity was significantly lower (-18 % for ABTS and -12 % for FRAP) in patients with metastasis as compared with patients without metastasis. The plasma total phenols, were also decreased (-16 %, p = 0.005) in metastasis patients. The patients with colorectal cancer have decreased total antioxidant capacity and the values are lower in patients treated with chemotherapy. Furthermore, CEA tumor marker levels are associated with a lower plasma total antioxidant capacity, and therefore with the progress and development of the disease.