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Importance: The American Joint Committee on Cancer (AJCC) has proposed the inclusion of pretreatment serum carcinoembryonic antigen (CEA) levels (C stage) into the conventional TNM staging system of colon cancer. The latter proposal has yet to be widely adopted because of the lack of long-term survival estimates of after C-stage incorporation into AJCC staging., Objectives: To evaluate whether long-term overall and cancer-specific survival is affected by inclusion of C stage into the standard AJCC TNM staging and to study the implications on survival estimates., Design, Setting, and Participants: We performed a retrospective study of all patients diagnosed as having histologically proven colonic adenocarcinoma from January 1, 2004, through December 31, 2005, from the National Cancer Institute's Surveillance, Epidemiology, and End Results database. We stratified each AJCC stage as C0 (normal) or C1 (elevated) based on the pretreatment serum CEA level. Median follow-up was 71 months., Main Outcome and Measures: Five-year estimates of overall and disease-specific survival and hazard ratios (HRs) for estimates of risk of overall and disease-specific mortality., Results: A total of 16 619 patients were evaluated, and of these, 8878 patients had C0 disease and 7741 had C1 disease. C1 stage was independently associated with a 51% and 59% increased risk of overall (HR, 1.51; 95% CI, 1.44-1.59; P < .001) and disease-specific mortality (HR, 1.59; 95% CI, 1.49-1.69; P < . 001) at a median follow-up of 71 months. Analysis of survival of the AJCC stages subdivided as C0 or C1 revealed a significant worse prognosis of C1 AJCC stages compared with the respective C0 AJCC stages. The magnitude of change in survival was large enough to cause clustering of survival estimates of C1 vs C0 cancers across various AJCC stages. Analysis of patients with stage I, II, and III cancer revealed that node-negative C1 disease was associated with prognosis similar or worse than node-positive C0 disease., Conclusions and Relevance: Inclusion of C stage into the AJCC TNM staging of colon cancer revealed significant differences dependent on C stage in terms of 5-year survival. C-stage inclusion resulted in substantial change in survival estimates, with C1 status portending a prognosis to certain stages similar to or worse than higher AJCC TNM stages with C0 status. We recommend routine pretreatment CEA testing as standard of care in colon cancer and use of C stage for multimodality treatment planning and risk stratification in prospective studies and randomized clinical trials.