Background: Immune Checkpoint Inhibitors (ICIs) changed the algorithm of treatment for Non-Small Cell Lung Cancer (NSCLC). Consequently, an increased incidence of specific immune-related adverse events (irAEs) were reported, although a clear relationship between toxicity and response to ICIs was not identified. To date no strong predictive factor of response to ICIs are univocally recognized in NSCLC. Methods: This is an observational, retro-prospective, monocentric study aimed to evaluate, in patients (pts) with NSCLC treated with ICIs, the predictive impact of selected blood biomarkers and the occurrence of irAEs on survival. Differential cells count, and specifically the Absolute Eosinophil Count (AEC, N°/μL), were registered at baseline to evaluate their impact on response to ICIs. Patients were divided in two groups: those with high AEC ( > 190/µL, group A), and those with low basal AEC ( < 190/µL, group B). Results: We enrolled 48 pts, median age was 70 years. All pts were treated with an ICI as single-agent: 5 (10%) with Nivolumab (N), 37 (77%) with Pembrolizumab (P), and 5 pts received Atezolizumab (A). Thirty-seven pts were treated in first-line setting, 9 pts in second-line, and 2 pts in successive lines. Of total, 63% pts (30/48) developed at least one irAE: an endocrine event occurred in 11 pts (23%), gastrointestinal toxicity in 10 pts (21%), dermatological events in 19 pts (40%), while a pulmonary irAE in 6 pts (13%). Progression free survival (PFS) was higher in group A than in group B (16 vs 9.5, p = 0.041) with a trend for a superior median Overall Survival (mOS) (median not reached vs 23 months, p = 0.07, respectively). At a median follow-up of 18 months (mos), pts who developed at least one irAE had a significant benefit in PFS (15mos vs 8mos, p = 0.027), reporting a trend for better PFS for pts with endocrine toxicities (p = 0.057) and GI- toxicities (p = 0.051) vs no toxicities. Pulmonary irAEs are significantly related to a better PFS: median not reached vs 12 mos, p = 0.025. Conclusions: In this real-life experience we observed that baseline AEC values correlates with PFS in pts with NSCLC treated with ICIs. Additional analyses are ongoing to identify appropriate cut-offs of AEC to better stratify patients. Moreover, our study underlines that the development of toxicities, in particular pulmonary ones, could be considered as a predictive indicator of response to ICIs in NSCLC. With all the limitations of being a small analysis, our real-life experience is innovative and explored prospectively the potential role of differential cell count and irAEs as predictive biomarkers of survival in NSCLC patients treated with ICIs.