BACKGROUND Immunotherapy targeting the programmed death-1 (PD-1)/programmed death ligand-1 (PD-L1) checkpoint has shown the good outcomes in non-small cell lung cancer (NSCLC). We investigated PD-1 and PD-L1 protein expression and their correlation with tumor-infiltrating lymphocytes (TILs), and association with survival in NSCLC. MATERIAL AND METHODS The expression of PD-1 (NAT105, Cell Marque) and PD-L1 (28-8, Dako) protein was assessed in 55 NSCLC cell lines by immunohistochemistry (IHC). PD-1 (NAT105, Cell Marque) and PD-L1 (22C3, Dako) protein expression was evaluated by IHC, and TIL percentage was scored, in 139 surgically resected specimens from patients with NSCLC. RESULTS PD-1 was not expressed on NSCLC cell lines. PD-L1 was expressed on 20 NSCLC cell lines (36.4%). A total of 60 patient samples (43.2%) were positive for PD-1 on the TILs, and 25 (18.0%) were positive for PD-L1 on tumor cells. High expression of PD-1 on tumor cells was significantly correlated with higher expression of PD-L1 (P=0.026) and a higher percentage of TILs (P<0.001). In the Cox regression model, the odds ratio for PD-1 was 2.828 (95% CI: 1.325-11.165; P=0.013) and 8.579 (95% CI: 4.148-22.676; P<0.001) when PD-L1 and TILs were positive. Patients whose tumor cells were PD-L1 negative had a tendency for longer relapse-free survival (RFS) than patients who were PD-L1 positive (1.85 years, 95% CI: 0.77-2.93 vs. 0.97 years, 95% CI: 0.71-1.23; P=0.054). CONCLUSIONS PD-1 was expressed on TILs in tumor tissues in NSCLC patients. PD-L1 was expressed on both TILs and tumor tissues. PD-1 expression was correlated with PD-L1 on tumor cells and TILs. Patients who were PD-L1 positive tended to experience progression after surgery.