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Background: In recent years, the anti-programmed cell death 1 (PD-1) drug pembrolizumab (Keytruda) was approved for treatment of unresectable advanced non-small cell lung cancer (NSCLC) as first- or second-line therapy depending on the clone 22C3-programmed death-ligand 1 (PD-L1) immunohistochemical expression score by the companion diagnostic assay. We herein evaluated 22C3-PD-L1 expression of NSCLC in a single institution experience and compared it with clinicopathologic features. Materials and methods: We assessed 22C3-PD-L1 expressions of 411 patients with NSCLC from our institution, including in past specimens. Programmed death-ligand 1 immunohistochemistry (IHC) testing was performed using the PD-L1 clone 22C3 pharmDx kit (Agilent Technologies/Dako, Carpinteria, CA, USA). Patients were separated into 3 groups with <1% (no expression), 1% to 49% (low expression), or ⩾50% (high expression) positive tumor cells. Results: In all, 137 patients (33%) did not express PD-L1, 155 (38%) showed low expression, and 119 (29%) demonstrated high expression. Archival samples showed lower PD-L1 expression than that of recent samples, and the ratios of no expression case significantly increased by using paraffin blocks embedded particularly in more than 4 years ago. Programmed death-ligand 1 positivity was significantly associated with male sex, smoking, higher tumor grade, squamous cell carcinoma in histologic type, wild-type EGFR, and ALK rearrangement positive. Conclusions: The rate of 22C3-PD-L1 expression of NSCLC detected in this study was similar to the frequencies of the previous reports, although the ratio of expression case decreased when using old paraffin blocks. [ABSTRACT FROM AUTHOR]