Abstract Background Refractory cardiogenic shock (CS) complicating new onset acute heart failure (new-AHF) is a medical emergency, requiring immediate mechanical circulatory support (MCS) to prevent hemodynamic deterioration. When non-invasive testing is impossible, endomyocardial biopsy (EMB) has an important role in determining the etiology of new-AHF. Purpose The aim of this study was to determine the procedural safety of EMB in patients with new-AHF requiring VA-ECMO. Additionally, we describe the detection rate of an underlying etiology of new-AHF on histopathological examination. Methods A retrospective, multicenter cohort of patients undergoing EMB while requiring VA-ECMO for new-AHF between 1990 and 2020 was compared with a cohort of non-transplant related biopsies not requiring VA-ECMO. Primary endpoint of the study was the safety of EMB with a secondary endpoint being the detection rate of an underlying etiology causing new-AHF on histopathologic examination. Results A total of 23 patients underwent EMB while requiring VA-ECMO (10.0%), 125 (54.3%) during an unplanned admission and 82 (35.7%) in elective setting. Major complications occurred in 8.3% of all procedures with a significantly higher rate in patients requiring VA-ECMO (26.1% vs. 8.0% vs. 3.7%,p=0.003), predominately due to occurrence of sustained ventricular tachycardia or need of resuscitation (13.0% vs. 3.2% vs. 1.2%,p=0.02). EMB led to a histopathological diagnosis in 78.3% of the patients requiring VA-ECMO which consisted primarily of patients with myocarditis (73.9%). Conclusions EMB in patients requiring VA-ECMO can be performed albeit with a substantial risk of major complications. This risk of the procedure was offset by a high detection rate of an underlying etiology on histopathological examination. The important therapeutic and prognostic implications of EMB in this population warrant further refinement to improve procedural safety. Funding Acknowledgement Type of funding sources: None. Major safety endpoints according to EMB