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The peripheral blood lymphocyte-to-monocyte ratio (LMR) has been associated with prognosis in many malignancies including metastatic melanoma. However, it has not been studied in patients treated with immune checkpoint inhibitors. In this study, we analyzed the baseline LMR with progression-free survival (PFS) and overall survival (OS) in metastatic melanoma patients treated with pembrolizumab. A total of 133 patients with metastatic melanoma treated with pembrolizumab were included in this retrospective study. LMR was calculated from pretherapy peripheral blood counts and the optimal cutoff value was determined by a receiver operator characteristic curve. PFS and OS were evaluated using the Kaplan–Meier method and multivariate Cox proportional hazard modeling. Patients with an LMR of at least 1.7 showed improved PFS (hazard ratio=0.55; 95% confidence interval: 0.34–0.92; P=0.024) and OS (hazard ratio=0.29; 95% confidence interval: 0.15–0.59; P=0.0007). The baseline LMR is associated with PFS and OS in metastatic melanoma patients treated with pembrolizumab, and could represent a convenient and cost-effective prognostic biomarker. Validation of these findings in an independent cohort is needed.