PURPOSE: Pembrolizumab is an effective front-line treatment for advanced non-small cell lung cancer (NSCLC) in patients expressing high levels of programmed death-ligand 1 (PD-L1). However, it is unclear whether first-line pembrolizumab has similar efficacy among elderly (aged ≥ 75 years) and younger patients. This study aimed to investigate the safety and efficacy of front-line pembrolizumab monotherapy in older adults with NSCLC expressing high PD-L1. METHODS: A total of 128 patients with advanced NSCLC expressing high PD-L1, including 47 older adults, received first-line pembrolizumab monotherapy at ten institutions in Japan, between February 2017 and February 2018. Data related to patient characteristics, efficacy of pembrolizumab therapy, and the type and severity of adverse events were recorded. RESULTS: Overall, 47 patients [40 men and 7 women; median age 79 (range 75-88) years] were included in our analysis. In patients who received first-line pembrolizumab monotherapy, overall response, disease control rates, median progression-free survival (PFS), and median overall survival (OS) were 53.1%, 74.4%, 7.0 months, and not reached, respectively. Common adverse events included anorexia, fatigue, skin rash, and hypothyroidism. Two treatment-related deaths were noted, due to pneumonitis and infection. First-line pembrolizumab monotherapy was associated with improved PFS in patients with non-progressive disease (PD). In patients with non-PD and good performance status (PS), pembrolizumab monotherapy improved OS. CONCLUSIONS: Elderly patients with NSCLC expressing high PD-L1 tolerated front-line pembrolizumab monotherapy well. Their survival outcomes were equivalent to those of younger patients. In patients with non-PD, first-line pembrolizumab monotherapy may improve PFS; in conjunction with good PS, it additionally improves OS.