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Metastatic melanoma (MM) is commonly treated with a combination of nivolumab and ipilimumab, regardless of tumor PD-L1 expression. We conducted a population-based study including all patients with MM (except ocular melanoma) treated in Denmark with first-line combination therapy or anti-PD-1 monotherapy since January 2017. Baseline data including known prognostic characteristics were used in multivariable and propensity-matched score (PMS) analyses to assess progression-free survival (PFS), melanoma-specific survival (MSS), and overall survival (OS) according to PD-L1 expression. We identified 1341 eligible patients, with known PD-L1 status for 1081 patients (43% PD-L1 ≥ 1%, 57% PD-L1 < 1%). PD-L1 ≥ 1% was an independent positive prognostic biomarker for survival in the overall cohort (MSS: HR 0.66, CI 0.52–0.83, p < 0.001). In the PMS PD-L1 ≥ 1% cohort, combination therapy showed similar clinical outcomes to monotherapy (PFS: HR 1.41, CI 0.94–2.11, p = 0.101; MSS: HR 1.21, CI 0.70–2.11, p = 0.49; OS: HR 1.17, CI 0.68–2.00, p = 0.567). In contrast, in the PMS PD-L1 < 1% and in the PMS PD-L1 < 1% BRAF WT cohorts, combination therapy improved PFS (respectively with HR 0.70, CI 0.53–0.93, p = 0.013; and HR 0.54, CI 0.37–0.78, p = 0.001), but did not reach statistically significant improvements of MSS (HR 0.72, CI 0.50–1.02, p = 0.065; and HR 0.79, CI 0.51–1.21, p = 0.278) or OS (HR 0.78, CI 0.56–1.08, p = 0.135; and HR 0.81, CI 0.54–1.21, p = 0.305) compared to monotherapy. Our findings support previous exploratory analyses of Checkmate-067, highlighting that improved clinical outcomes with combination therapy are not established in unselected patients with high (≥1%) tumor PD-L1 expression. • PD-L1 ≥ 1% is an independent positive prognostic biomarker in metastatic melanoma • PD-L1 ≥ 1% did not derive additional benefit from anti-CTLA-4 over anti-PD-1 alone • Combination therapy demonstrated improved PFS only in patients with PD-L1 < 1% [ABSTRACT FROM AUTHOR]