BACKGROUND: The role of pembrolizumab in the treatment of poor performance status (PS) patients remains unclear. PATIENTS AND METHODS: We conducted a phase II trial to investigate the efficacy and safety of pembrolizumab as first-line therapy for non-small-cell lung cancer (NSCLC) patients with PSs of 2-3 and programmed cell death ligand 1 (PD-L1) expression ≥ 50%. The primary endpoint of this study was the objective response rate (ORR). RESULTS: Fourteen patients treated at eight institutions were enrolled. Most patients had PS 2 (12/14; 86%) and others had PS 3 (2/14; 14%). The ORR was 57.1% (95% confidence interval 28.9-82.3%), which met the primary endpoint. The median progression-free survival (PFS) and 1-year PFS rates were 5.8 months and 20.0%, respectively. At the time of data cut-off, one patient had received treatment for more than 1 year; another patient had received treatment for more than 2 years. Nine patients had improved PS with treatment (Wilcoxon signed-rank test, p = 0.003). Two patients had immune-related adverse events ≥ grade 3: grades 5 and 3 elevation in alanine and aspartate aminotransferases. Two PS 3-stage patients were diagnosed with clinically progressive disease prior to initial computed tomography; both died within 2 months. CONCLUSION: Pembrolizumab was effective for the treatment of NSCLC patients with a poor PS and PD-L1 level ≥ 50%. However, given the poor outcomes of the PS 3 patients, the drug is not indicated for such patients. Adverse events, including liver dysfunction, should be carefully monitored. REGISTRATION ID: UMIN000030955.