ABSTRACT Introduction: Immunotherapy has become a standard treatment for lung cancer; the objective of this study was to evaluate the effectiveness, safety of pembrolizumab monotherapy in patients with advanced or metastatic Non-Small Cell Lung Cancer (NSCLC) used in real-world clinical practice. Material and methods: Retrospective observational study of every patients treated with pembrolizumab in our centre from January 2017 to June 2019. Outcomes collected: sex, age, ECOG, PDL-1 levels, previous metastatic line therapies, adverse events (AE) and smoking status. Results: A total of 62 patients were reviewed. The median age was 62.34±10.62 years, 48 (77.41%) were men and 91.93% of patients had ECOG 0. The median doses administered was 170.5 mg (108-240 mg) and median follow-up was 3 months (range: 1-38). A median of 4 cycles of pembrolizumab (range 1 to 56) were administered as monotherapy. The reason for treatment discontinuation was mainly due to disease progression in 38.70% patients or death in 30.64%. As first-line pembrolizumab monotherapy, median progression free survival was 7.7 months (95% CI: 3.66-11.73) (N=33). With respect to patients who were treated in second-thirdline treatment, median PFS was 3.5 months (95%CI: 2.40-4.59) (N=29). As to overall survival, pembrolizumab-treated patients as first-line treatment reached 19 months median OG (95% CI: 13.36-24.63) (N=33) and those treated in second-third-line treatment got 11 months (95% CI: 3.4-18.5). 64.51% of patients presented some AE to pembrolizumab however, only, 9.38% of them were grade 3. Conclusion: Pembrolizumab represents an effective and feasible alternative in terms of SLP. It is a well-tolerated treatment option.