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Background: Although clinical trials have investigated the addition of pembrolizumab to chemotherapy for non‐small cell lung cancer, none have investigated the addition of chemotherapy to pembrolizumab. Methods: We conducted a retrospective study of 71 NSCLC patients including 33 treated with pembrolizumab plus chemotherapy (combination therapy group) and 38 treated with pembrolizumab monotherapy (monotherapy group) from 1 May 2016 to 31 August 2020. Results: Eleven of 33 (33.3%) patients in the combination therapy group and 37 of 38 (97.4%) patients in the monotherapy group had programmed cell death ligand‐1 (PD‐L1) tumor proportion score (TPS) ≥50%. Objective response rate (ORR) and median overall survival (OS) were not significantly different between the combination therapy group and monotherapy group (54.5% vs. 47.4, p = 0.637 and 16.6 vs. 27.0 months, p = 0.463). In patients with PD‐L1 TPS ≥50%, ORR and median OS were not different between the combination therapy group and the monotherapy group (63.6% vs. 48.6%, p = 0.499 and not reached vs. 27.0 months, p = 0.976). Thirty‐three (100%) patients experienced adverse events (AEs) in the combination therapy group and 32 (84.2%) in the monotherapy group. Treatment discontinuation at 1 year due to AEs occurred more frequently in the combination therapy group (45.2%) than in the monotherapy group (21.1%). Conclusion: There was no significant difference in ORR and OS between the two groups, and treatment discontinuation was more frequent in the combination group. A randomized controlled trial is needed to evaluate the addition of chemotherapy to pembrolizumab for first‐line treatment in patients with PD‐L1 TPS ≥50%. [ABSTRACT FROM AUTHOR]