In general a rising carcinoembryonic antigen (CEA) level means tumor progression. We observed a transient increase in CEA level despite objective response among patients receiving chemotherapy for metastatic colorectal cancer. This surge phenomenon has not previously been described for patients with metastatic colorectal disease. CEA was measured every second week in 27 patients receiving oxaliplatin, 5-fluororuracil, and folinic acid as first-line therapy against metastatic colorectal cancer. Four patients (15%, 95% CI 5-31%) met the criteria for therapy-induced CEA surge. The time of reaching maximum CEA level varied from 13 to 56 days. Median rise in CEA from baseline was 263% (range 24-632%). An initial rise of CEA during chemotherapy in colorectal cancer patients may therefore not always indicate progression of disease but may be a transient CEA surge in patients responding to chemotherapy.