Results from the phase III clinical trial established pembrolizumab monotherapy as the first-line standard of care for patients with metastatic NSCLC who have PD-L1 expression ≥ 50%, , and wild-type tumors. However, given the differences between patients treated in routine clinical practice and those treated in a clinical trial, real-world data are needed to confirm the treatment benefit in standard practice. Given the lack of data on large cohorts of patients with long follow-ups, we designed an observational retrospective study of patients with metastatic NSCLC who were treated with pembrolizumab, starting from its reimbursement eligibility until December 2020. The primary endpoints were PFS and OS, determined using the Kaplan-Meier method. Response and safety were also evaluated. We followed 880 patients (median follow-up: 35.1 months) until February 2022. Median PFS and OS were 8.6 months (95% CI: 7.6-10.0) and 25.5 months (95% CI: 21.8-31.6), respectively. We also found that ECOG PS, PD-L1 expression, and habitual smoking were prognostic factors for PFS, while age, sex, ECOG PS, habitual smoking and histology had an impact on OS. Multivariable analysis confirms the prognostic role of PD-L1 for PFS and of ECOG for both PFS and OS. 39.9% of patients reported an adverse event, but only 6.3% of patients discontinued therapy due to toxicity. Our results suggest a long-term benefit of pembrolizumab in the first-line setting, as well as a safety profile consistent with the results of . Many collected variables appear to influence clinical outcome, but results from these exploratory unadjusted analyses should be interpreted with caution.