Mesothelioma is a neoplasm which may arise from the serous membranes of the thorax, abdomen, or genital tract. Two types are recognized: the solitary, fibrous type which may be benign or malignant, and the diffuse, spreading type which is invariably malignant. Wagner (19) in 1870 was probably the first to recognize mesothelioma as a specific entity, when he described a patient with a “tubercle-like lymphadenoma” of the chest. The first reference to mesothelioma in the American literature appeared in 1891, when Biggs (3) published two cases diagnosed as “endothelioma of the pleura.” Subsequently, innumerable references to this tumor have been made under a wide variety of designations.2 Most reports are of single cases. The largest series thus far reported are 24 cases by Clagett et al. (5) and a collection of 27 cases of the diffuse, spreading type by Poulsen and Sorensen (11). Until 1942 the diffuse spreading type of mesothelioma was recognized as a malignant tumor of the pleura, but there was no unanimity of opinion regarding the cell of origin of the localized, fibrous type. In 1942, Stout and Murray (18) pinpointed the identity of this latter lesion. They proved by tissue culture technics the mesothelial origin of a solitary tumor which was broadly attached to the pleura at the periphery of a lung and was composed of spindle-shaped cells. The purpose of the present publication is to emphasize the fact that mesothelioma occurs with greater frequency than may be realized and should be considered in the differential diagnosis of all intrathoracic masses shown radiographically to be in close relationship to the pleura. Pathology In 1950 Stout (16) reviewed the pathology of mesothelioma and separated these neoplasms into three groups: (a) the solitary, fibrous type, which may be benign or malignant; (b) the diffuse, spreading, highly malignant type; (c) the benign mesothelioma of the male and female genital tracts. Intrathoracic mesothelioma may be solitary or diffuse and may arise from either the visceral or parietal pleura or the pericardium. Grossly the localized mesothelioma tends to be a firm, encapsulated, yellow tumor which may be highly vascular, with prominent veins over its external surface. Microscopically the benign localized tumor consists of spindle-shaped cells which are uniform in appearance and are located within a collagen and reticulum matrix (1). Malignant localized mesothelioma is characterized histologically by larger, non-uniform cells with numerous mitoses and by areas of hemorrhagic necrosis, as well as a paucity of collagen and reticulum matrix. The diffuse, malignant type of mesothelioma is characterized by grossly thickened pleura or pericardium studded with many nodular densities. The tumor may spread by direct extension and serosal seeding to involve the lung, pericardium, mediastinum, diaphragm, and chest wall (8). Regional and distant lymph-node and organ metastases may occur.