BACKGROUND. Immunotherapy with immune checkpoint inhibitors (ICIs) recently became the standard treatment for advanced non-small cell lung cancer (NSCLC) patients. Here, we present the first results of a real-world observational study on the effectiveness of ICI monotherapy in advanced NSCLC patients treated at a single academic centre in a Central and Eastern European (CEE) country. METHODS. Overall, 66 consecutive advanced NSCLC patients treated with ICIs in everyday clinical practice, either with first-line pembrolizumab (26 patients) or second-line atezolizumab, nivolumab or pembrolizumab (40 patients), from August 2015 to November 2018, were included. All data were retrieved from a hospital lung cancer registry, where the data is collected prospectively. RESULTS. Included patients had a median age of 64 years, most were male (55 %), 6 % were in performance status ≥ 2, and 18 % had controlled CNS metastases at baseline. In first-line, the median progression-free survival (mPFS) was 9.3 months, while the median overall survival (mOS) was not reached. The 1-year OS was 62 %. In second-line, the mPFS and mOS were 3.5 months and 9.9 months, respectively, with a 1-year OS of 35 %. In the overall population, adverse events of any grade were recorded in 79 % of patients and of severe grade (3 -4) in 12 % of patients. CONCLUSION. The first real-world outcomes of NSCLC immunotherapy from a CEE country suggest comparable effectiveness to those observed in clinical trials and other real-world series, mainly coming from North America and Western European countries. Further data to inform on the real-world effectiveness of immunotherapy worldwide are needed. The Oncologist 2021;9999:• • Implications for Practice: Following the results from pivotal trials, immunotherapy became a standard treatment of advanced NSCLC. The real-world data on immunotherapy effectiveness in everyday clinical practice is still limited. This article presents the first data on the effectiveness of mono-immunotherapy with ICIs for patients with advanced NSCLC treated at a single academic centre in a CEE country. The survival rates and observed toxicity are comparable to those achieved in randomized clinical trials and other real-world series, coming mainly from North America and Western European countries. There is a pressing need to gather further data on the effectiveness of immunotherapy in everyday practice worldwide.