The American Joint Committee on Cancer (AJCC) has proposed the inclusion of pretreatment serum carcinoembryonic antigen (CEA) level (C-stage) into the conventional TNM staging system of colon cancer. We assessed the prognosis of various stages of colon cancer after such an inclusion. Data for all patients (N = 17 910) diagnosed with colonic adenocarcinoma (AJCC stages I, IIA, IIB, IIC, IIIA, IIIB, IIIC, and IV, based on TNM staging system) between January 1, 2004, and December 31, 2004, with a median follow-up of 27 months (range 0-35 months), were collected from the Surveillance, Epidemiology, and End Results database. C-stage (C0-stage = normal CEA level; C1-stage = elevated CEA level) was assigned to all patients with available CEA information (n = 9083). Multivariable analyses using Cox proportional hazards models were used to identify independent factors associated with prognosis. Prognosis of overall stages (AJCC stages I-IV and C0 or C1) was analyzed using Kaplan-Meier survival curves. All statistical tests were two-sided. C1-stage was independently associated with a 60% increased risk of overall mortality (hazard ratio of death = 1.60, 95% confidence interval = 1.46 to 1.76, P < .001). Overall survival was decreased in patients with C1-stage cancer compared with C0-stage cancer of the respective overall stages (P < .05). Similarly, decreased overall survival was noted in patients with stage I C1 cancer compared with stage IIA C0 or stage IIIA C0 cancer (P < .001), in patients with stage IIA C1 cancer compared with stage IIIA C0 (P < .001), and in patients with stage IIB C1 or stage IIC C1 cancer compared with stage IIIB C0 cancer (P < .001). C-stage was an independent prognostic factor for colon cancer. The results support routine preoperative CEA testing and C-staging upon diagnosis of colon cancer and the inclusion of C-stage in the conventional TNM staging of colon cancer.