A 3.5-month-old infant with the diagnosis of imperforated anus and hypospadias was referred to our clinic with a history of colostomy in the neonatal period. On physical examination, he was acyanotic with normal cardiac auscultation. Electrocardiogram and chest X-Ray were normal. On echocardiography, an echo-free space was seen behind the left ventricular posterior wall, both in the parasternal long-axis and modified four-chamber views (
Figure 1 echo videos 1 and 2 in the supplementary file). There were tissue strands within this space, mimicking the presence of purulent pericardial effusion. Lowering the Nyquist frequency revealed blood flow in this huge effusion-like, echo-free space (
Figure 2). Thorough examination showed an unusually shaped congenital giant right atrial aneurysm (CGRAA), with an oblique lie, extending from the right side and anterior of the heart to the left and posterior of the left ventricle (Videos 3 - 5 in the supplementary file). The infant experienced episodes of witnessed transient supraventricular tachycardia during echocardiographic examination and afterwards during admission. He underwent surgical operation for resection of aneurysm (Video 6 in the supplementary file). Immediately, after weaning from cardiopulmonary bypass (CPB), he developed hypotension. The CPB was re-established. Posterior descending coronary artery (PDA) was damaged and the patient died in the operation room.