Carcinoembryogenic antigen (CEA) is useful in the evaluation of chemotherapy response of metastatic colorectal cancer (CRC). We studied weekly CEA during one fluorouracil-based chemotherapy cycle, correlated with long-term (8-12 week interval) computed tomography (CT) and CEA responses.CEA, liver function tests and inflammatory parameters were measured prospectively at baseline, day 7, day 14, and after the cycle (day 21/28), in 60 patients with metastatic CRC.CEA non-significantly decreased at day 7 and was increased on day 14. In progressive disease, CEA increased significantly during the evaluation cycle (55.4 μg/l vs. 148.2 μg/l; p=0.024), but the level was stable in patients with disease control (10.6 μg/l vs. 17.8 μg/l; p=0.58). CEA fluctuation correlated neither with liver function test nor with inflammatory parameters. Correlation of long-term response was most evident in progressive disease.CEA should not be measured during 5-fluorouracil-based oral chemotherapy nor within two weeks from intravenous chemotherapy administration.