Mediastinal foregut duplication cysts are rare congenital masses caused by developmental disorders of the anterior part of the embryonic primitive gut. In adults they can be discovered as an incidental finding on chest radiograph. They can mimic other intrathoracic pathologies as it was initially the case in our patient. A 51-year-old woman was incidentally found to have homogenous mass at the right cardiophrenic angle on the chest radiograph. Based on that finding and echocardiography a pericardial cyst was suspected. Computed tomography demonstrated a simple anterior mediastinal cyst. Because of the growth with a mild progression of the compression of the right atrium, the cyst was resected. Histology and immunohistochemistry revealed the mass to be a mediastinal foregut duplicatory cyst of enteric type containing persistent thymus. To our knowledge, this is the first report in which a mediastinal foregut cyst contained both enteric mucosa and ectopic persistent thymus (Fig. 9, Ref. 13). BACKGROUND foregut cyst, mediastinal cyst, thymus, enteric cyst, pericardial cyst.