We report the case of a patient with ischaemic cardiomyopathy who was admitted to our hospital for an echocardiographic follow-up a few days after the implantation of a biventricular pacemaker/defibrillator. Transthoracic echocardiographic examination revealed an immobile echogenic mass attached to the left atrial side of the fossa ovalis which was not present a month before the biventricular pacemaker implantation. Because the images of the pre-operative echocardiogram were not available at the time of the examination, the differential diagnosis included a tumour or a thrombus formed on the left side of intra-atrial septum. The use of low myocardial infarction contrast echocardiography and power Doppler clarified the lack of microcirculation and blood flow within the mass, respectively, and confirmed the diagnosis of thrombus. After an episode acute renal failure, the thrombus was absent at a follow-up echocardiogram presumably because of peripheral embolization.