Recent experiences with lesions present in the anterior mediastinum led to a review of the cases of pericardial celomic cyst recorded in this department in an attempt to reach some conclusion as to what diagnostic procedures are justified beyond proving that such a cyst does, in truth, arise in the thorax and is not an inoperable malignant growth. This investigation was stimulated by Funch and Wenger's (1) advocacy of tapping of cystic lesions and injection of air for contrast studies and for examination of the fluid contents. There was also a feeling that the exact designation of a cystic lesion preoperatively was little more than an academic exercise and that, for that reason, too zealous pursuit might be unwarranted. Many authors (2–6) have pointed out the difficulty of the roentgen diagnosis of pericardial celomic cyst among the mass lesions in the anterior inferior mediastinum. Cooper, Archer, and Mapp (6) enumerated eleven other possibilities in this location: fat pad, lipoma, dermoid, teratoma, neurofibroma, diaphragmatic hernia, eventration of the diaphragm, cardiac aneurysm, sacculated pericarditis, primary pulmonary neoplasm, and metastatic cancer. Laipply (7) lists eight types of congenital cyst which may be present in this area: epidermoid, dermoid, teratoid, bronchial, esophageal, gastroenteric, pericardial celomic, and lymphangiomatous. This problem did not become important until Pickhardt (8), in 1934, reported the successful removal of a cyst from the chest of a fifty-three-year-old woman who complained of sharp, knife-like pericardial pain, and whose chest film showed a circular shadow just above the left diaphragm. Lambert (9), in 1940, by a process of exclusion, established the entity of pericardial celomic cyst and presented a theory as to its origin. He postulated the persistence, as a separate cavity, of one of the mesenchymal lacunae which, in the developing embryo, fuse to form the pericardial celom. Lillie et al. (4) have since expanded this theory to explain the formation of pericardial diverticula and of lesions diagnosed as pericardial celomic cyst which were found more superiorly situated in the anterior mediastinum than the cardiophrenic sulcus. Lambert, in his process of exclusion, indicated that the distinction between pericardial celomic and lymphangiomatous cysts was not to be made on a histologic basis, since it was impossible to distinguish between mesothelium and endothelium. He implied, therefore, that gross appearance must be used in differentiation. He described lymphangiomatous cysts as multilocular, and intimately associated with the structures with which they are in contact (i.e., adherent). He stated further that attempts at their removal are accompanied by profuse bleeding because of the multiple sources of the blood supply. Several writers have used Lambert's article as a basis for their reports. Apparently, Funch and Wenger's advocacy of tapping anterior mediastinal cysts stems from this.