The purpose of the present study was to retrospectively analyze the clinicopathological characteristics and clarify whether or not the preoperative carcinoembryonic antigen (CEA) level could be used as a decision-making factor as an adjunct to the TNM staging system in patients with clinical stage I non-small cell lung cancer (NSCLC). Between 1993 and 2006, 815 patients who had clinical stage I NSCLC were analyzed retrospectively. The CEA level was defined as being either normal (CEA<or=5 ng/ml), high (5<CEA<or=30 ng/ml) and very high (CEA>30 ng/ml) sub-groups. The rate of patients with an elevated CEA level was 33.6%. The five-year disease-free survival rates for patients with normal, high and very high CEA levels were 76.7, 60.0 and 31.3%, respectively. The survival curve for patients with a normal CEA level almost overlapped that for p-stage I, that for a high CEA level nearly overlapped that for p-stage II, and that for a very high CEA level nearly overlapped that for p-stage III. The present study demonstrated that the preoperative CEA level was a very good predictor of the pathological stage. These findings suggest that the preoperative CEA level may be useful as an adjunct to the TNM staging system.