This study aimed to establish the clinical significance of preoperative serum cytokeratin 19 fragment (CYFRA21-1) and Sialyl Lewis(x) (SLX) in patients with stage I non-small cell lung cancer (NSCLC). The study involved 137 patients (87 male, 50 female; median age 69 years) with completely resected stage I NSCLC. SLX, carcinoembryonic antigen (CEA), squamous cell carcinoma antigen (SCC), and CYFRA21-1 were examined. Receiver operator characteristic (ROC) curves were constructed to determine prognostic cut-off values. Among the 137 patients, we identified 30 with recurrence within 3 years. The 5-year survival rates in patients with (n=30) and without (n=107) recurrence were 14% and 81%, respectively. The serum concentrations of SLX, CEA, and CYFRA21-1 in the recurrence group were significantly higher than those in the non-recurrence group. The areas under the ROC curve (AUC) were 0.72, 0.65, 0.53, and 0.64 for SLX, CEA, SCC, and CYFRA21-1, respectively. The prognostic cut-off values were 36U/ml, 7.8ng/ml, 1.5ng/ml, and 3.2ng/ml for SLX, CEA, SCC, and CYFRA21-1, respectively. A log-rank test revealed that age, performance status, T factor, lymphatic invasion, vascular invasion, SLX, CEA, SCC, and CYFRA21-1 were all significantly associated with survival. By multivariate analysis, age, performance status, lymphatic invasion, SLX (risk ratio, 4.11) and CYFRA21-1 (risk ratio, 3.47) were independent prognostic factors. For patients positive for both CYFRA21-1 and SLX, the relative risk was 5.32 compared with patients who were negative for both markers. The 5-year survival rates were 80% in the group negative for both markers (n=86); 52% in the group positive for one of the markers (n=43); and 13% for the group positive for both markers (n=8) (p<0.001). We concluded that serum SLX and CYFRA21-1 were prognostic markers for stage I NSCLC. Their combination should contribute to the classification of stage I NSCLC patients. There is a need to consider adjuvant and neoadjuvant therapies to improve prognosis in patients positive for both tumor markers.