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The immune regulatory enzyme indoleamine-2,3-dioxygenase (IDO-1) suppresses T cell responses and may reduce efficacy of therapies targeting immune checkpoints such as programmed death receptor-1/programmed death ligand-1 (PD-1/PD-L1). Early phase clinical trials combining IDO-1 and PD-1/PD-L1 inhibitors have shown some promise in non–small cell lung cancers (NSCLCs). However, the coexpression of IDO-1 and PD-L1 has not been thoroughly investigated, and the potential for IDO-1 immunohistochemical expression as a therapeutic biomarker is unknown. One hundred two cases of NSCLC (51 adenocarcinomas, 9 adenosquamous carcinomas, and 42 squamous cell carcinomas) were evaluated for IDO-1 and PD-L1 expression by immunohistochemistry. IDO-1 expression was identified in 43% of NSCLC (42% of adenocarcinomas, 44% of adenosquamous carcinomas, and 43% of squamous cell carcinomas). Coexpression with PD-L1 (≥1%) was common (27% overall; 27% of adenocarcinomas, 33% of adenosquamous carcinomas, and 26% of squamous cell carcinomas). A smaller population of tumors showed isolated PD-L1 (25% overall; 16% of adenocarcinomas, 44% of adenosquamous carcinomas, and 33% of squamous cell carcinomas) or IDO-1 expression (15% overall; 14% of adenocarcinomas, 11% of adenosquamous carcinomas, and 17% of squamous cell carcinomas). In summary, IDO-1 is commonly expressed by NSCLC, and its frequent coexpression with PD-L1 may account for the increased efficacy seen with dual blockade of PD-1/PD-L1 and IDO in clinical studies. IDO-1 immunohistochemistry may be a useful biomarker for selection of patients who could benefit from dual-agent therapy and should be evaluated in prospective clinical trials using PD-1/PD-L1 and IDO inhibitors.