Imaging. Preoperative posterioranterior (A) and lateral (B) chest X-ray showing a large mass in the anterior mediastinum with posterior displacement of the heart. Transthoracic echocardiography (C) showing a large echolucent space around the heart (parasternal long-axis view). Computertomography (D) showing a large cyst in the anterior mediastinum encasing the heart with compression of both lungs (transverse view at the level of the aortic valve). Gross examination (E) revealed a thin-walled cyst filled with clear fluid. A postoperative posterioranterior chest X-ray (F) showed a remarkable improvement and a normal cardiac silhouette. Microscopic histopathologic examination (G) showed a cyst wall lined by cubic cells and underlying loose connective tissue with remnants of thymic tissue.