BACKGROUND: Pembrolizumab is currently approved as a first-line therapy for advanced non-small cell lung cancer (NSCLC) patients with a programed death ligand-1 (PD-L1) expression ≥50%. However, the association between the efficacy of pembrolizumab and PD-L1 expression levels in patients with PD-L1 expression ≥50% has not been fully elucidated. METHODS: We retrospectively analyzed patients with advanced NSCLC and a PD-L1 tumor proportion score (TPS) of ≥50% who received pembrolizumab as a first-line therapy at 11 institutions in Japan between February 2017 and January 2018. Patients were divided into TPS 50-89% and TPS 90-100% (ultra-high PD-L1 expression) cohorts. RESULTS: In total, 149 patients were included: 99 (66.4%) and 50 (33.6%) patients were in the TPS 50-89% and TPS 90-100% cohorts, respectively. Baseline characteristics were similar between the TPS 90-100% and TPS 50-89% cohorts. The objective response rates (ORR) in the TPS 90-100% and TPS 50-89% cohorts were 58.0% and 46.5%, respectively (p = 0.23). Time to treatment failure (TTF) was longer in the TPS 90-100% cohort than in the TPS 50-89% cohort (hazard ratio [HR]: 0.67, 95% confidence interval (CI): 0.42-1.07; p = 0.09). Although TTF within 120 days after the initiation of pembrolizumab therapy was comparable between both cohorts (p = 0.54), TTF after 120 days was significantly longer in the TPS 90-100% cohort than in the TPS 50-89% cohort (HR: 0.22, 95% CI: 0.06-0.87; p = 0.031). Immune related adverse events of grade 3 or more occurred in 16.0% and 19.2% of patients in the TPS 90-100% and TPS 50-89% cohorts, respectively. CONCLUSIONS: The patients with an ultra-high PD-L1 expression continued pembrolizumab therapy longer, driven by a reduced risk of treatment failure in the late phase. PD-L1 expression levels might be a predictive biomarker of a first-line immunotherapy benefit in the late phase among NSCLC patients with TPS ≥50%.