Thromboembolism in patients receiving extracorporeal membrane oxygenation (ECMO) support is a feared complication. Systemic anticoagulation during ECMO in patients with a massively dilated left ventricle (LV) and decreased LV systolic function is still debated. Hearin, we report a case of a 5-month old infant on ECMO support who had fatal thrombus formation in the massively dilated LV and a consequent thromboembolic event.