Abstract Background Immune checkpoint inhibitors (ICIs) have revolutionized the clinical outcomes of cancer. Nevertheless, the response varies across populations and their use may lead to ICIs-related adverse events (irAEs), including cardiovascular (CV) irAEs, which can be life-threatening. Limited evidence exists regarding the safety and efficacy of ICIs therapy in patients presenting with pre-treatment reduced left ventricular ejection fraction (LVEF). Objectives To evaluate the safety and efficacy of ICIs therapy in patients with pre-treatment reduced LVEF. Methods Retrospective single center cohort of patients treated with ICIs therapy, who performed pre-treatment LVEF assessment. The primary endpoint was to evaluate the safety of ICIs among this population, assessed by CV irAEs (composite of myocarditis, acute coronary syndrome, heart failure, and arrhythmia). The secondary endpoint was to evaluate the efficacy of ICIs, assessed by all-cause mortality and progression-free survival (PFS). Results The cohort included 307 patients with 30 (10%) patients presenting with pre-treatment reduced LVEF, with a mean LVEF of 39±7%. While a significantly higher prevalence of CV irAEs was observed in the reduced LVEF group (37% vs. 14%, p=0.004), following a multivariable cox regression analysis including baseline cardiovascular diseases and risk factors, pre-treatment reduced LVEF did not remain a significant predictor (p=0.635). No significant differences were observed between the groups regarding all-cause mortality and PFS. Conclusions Pre-treatment reduced LVEF was found to be safe with no significant impact on efficacy among patients treated with ICIs.