Carcinoembryonic antigen and CA 19.9 are markers of colorectal neoplasms, often related to tumor burden. Elevated pre-operative levels parallel disease extent and may foresee adverse prognosis. CEA and CA 19.9 may increase post-operatively, indicating tumor recurrence. Only limited data are available on the influence of chemotherapeutic treatments on these two markers, to detect chemotherapy-induced cytolysis. We measured CEA and CA 19.9 before and after a chemotherapy regimen of five days consisting of folinic acid and 5-fluorouracil in forty patients with colorectal cancer. No consistent fluctuation was detected, the examined tumor markers being related in a given patient only to tumor burden.