We retrospectively screened 108 patients with advanced non-small cell lung cancer (NSCLC) treated via S-1 monotherapy…. Sixty-one patients with high CYFRA 21-1 levels had a statistically significant shorter progression-free survival (PFS) and overall survival (OS) than 46 patients with normal levels (median PFS = 42 days vs. 70 days, respectively; p = 0.0014; median OS = 197 days vs. 316 days, respectively, p = 0.0239)....Serum CYFRA 21-1 levels have predictive and prognostic roles in the management of patients with advanced NSCLC on S-1 monotherapy.