We aimed to investigate the prognostic significance of serum human epididymis protein 4 (HE4) level in patients with locally advanced non-small cell lung cancer (LA-NSCLC) who underwent definitive chemo-radiotherapy (CRT). A hundred seventeen patients with the diagnosis of LA-NSCLC were enrolled. The serum concentrations of HE4 were measured at the beginning of CRT, at the end of CRT, and 3 months after the completion of CRT. The median follow-up period was 21.7 months (range, 5.4-39.8 months). The mean serum HE4 levels prior to CRT, at the end of the CRT, and 3rd month after the completion of CRT were 159.2, 130.2, and 127.5, respectively (p= 0.023). The median progression free survival (PFS) was 15.4 months. One, and two-year PFS rates were 58.1%, and 22.2%, respectively. One, and two-year expected survival rates were 81.2%, and 62%, respectively. In multivariate analysis, stage (p= 0.002), HE4 levels after 3 months of CRT (p= 0.037) were predictive of OS. Stage IIIC patients had 10.2 times likely to death when compared to stage IIIA patients (95%CI: 2.3-45.7; p= 0.037). The increase of 1 HE4 levels after 3 months of CRT increased the mortality rate 1.002 (95%CI: 1.000-1.0004; p= 0.037). In multivariate analysis stage was predictive of PFS. When compared to stage IIIA patients, stage IIIC patients have 2.5 times risk for progression (95% CI: 1.2-5.2; p= 0.014). Our findings suggested that serum HE4 may be an important prognostic biomarker for LA-NSCLC patients. This issue warrants further prospective studies with more patient populations.