We describe an unusual case of extracardiac mass in a patient who was assymptomatic and submitted to an open-chest coronary artery bypass operation 5 years previously. Transthoracic and transesophageal echocardiography revealed a rounded mass compressing the right atrium with a thick contour surrounding a heterogeneous center, suggestive of foreign body. Magnetic resonance imaging identified the mass as a pericardial cyst. Operation revealed a fibrinous cyst encapsulating a sponge. This case illustrates the central role of echocardiography for the diagnosis and treatment of patients with suspected retained surgical sponges.