Purpose: Immunotherapy is promising for lung cancer treatment, although at significant financial impact. The aim of this study was to evaluate the effectiveness and the efficacy-effectiveness gap of pembrolizumab in previously treated non-small cell lung cancer (NSCLC). Methods: A population-based ambispective cohort study was conducted. Cases of interest were identified through the National Cancer Registry database and additional data sources. Patients aged ≥18 years, diagnosed with NSCLC and exposed to pembrolizumab, between 23 June 2016 and 31 October 2018, as second or later lines of treatment for advanced disease were included. Patients were followed-up until death or cutoff date (30 April 2019). Primary outcome was overall survival (OS). Secondary outcomes were progression-free survival (PFS), event-free survival (EFS), and adverse events (AEs) leading to treatment discontinuation. The efficacyeffectiveness gap was evaluated comparing results with clinical trial data. Results: A total of 181 patients were included. Median age was 63 years (range 33-94); 74.6% were male. Median treatment duration was 5.6 months (interquartile range: 1.4-10.4) and, at cutoff date, treatment had been discontinued in 141 patients, mainly due to disease progression. Median OS was 13.0 months (95% confidence interval [CI] 9.3-15.9) and 1-year OS was 53.1% (95% CI 45.2%-60.3%). Median PFS was 5.6 months (95% CI 4.6-7.2), median EFS was 4.7 months (95% CI 3.2-6.0), and treatment was discontinued due to AE in 8.3% of cases (n = 15). The efficacy-effectiveness gap seems to favor pembrolizumab use in clinical practice. Conclusion: Real-world data suggest the performance of pembrolizumab to reflect the clinical trial outcomes in previously treated NSCLC.