OBJECTIVE We evaluated preoperative serum carcinoembryonic antigen (CEA) as a prognostic factor for colorectal cancer and determined when surveillance of this marker was useful. METHODS Serum CEA was measured preoperatively in 1,263 patients who underwent curative resection for colorectal cancer at 3-month intervals for the first 2 postoperative years and at 6-month intervals thereafter. Mean follow-up was 48 months (range 1-156 months). RESULTS The 5-year disease-free survival was less in patients with a high preoperative serum CEA level (P<0.0001). Among patients with a tumor recurrence, 38.5% had high follow-up serum CEA levels. The number of patients with high postoperative serum CEA levels exceeded the number of patients with high preoperative levels. High preoperative and follow-up serum CEA levels were independent prognostic factors for tumor recurrence (P=0.003 and P<0.001, respectively). In patients with high preoperative serum CEA levels, CEA surveillance had a 92.3% positive predictive value (PPV) and a 96.1% negative predictive value (NPV). The mean interval between postoperative serum CEA elevation and the diagnosis of a tumor recurrence [diagnostic interval (DI)] was 2.5 months (range 5-17 months). The DI was 0 in 18.8% of patients with a tumor recurrence. CONCLUSIONS High serum CEA levels preoperatively and at follow-up are prognostic factors for colorectal cancer. Postoperative serum CEA surveillance is used most effectively when patients have high preoperative serum CEA levels. Considering the DI of 0 in 18.8% of the patients, the current CEA surveillance schedule might be changed.