Mesothelioma of the pleura is a neo-plastic disease which develops from the lining cells of the pleura. Its origin has long been in dispute, but the recent tissue culture studies of Stout and Murray (1), confirmed by Sano, Weiss, and Gault (2), have established the mesothelial char-acter of the tumor. The controversy regarding mesothelioma has led to more than thirty designations (3) , due to the fact that mesothelial cells may assume different histologic forms. The term endothelioma has frequently been used (4) because it was believed that the tumor originated from endothelial structures. Robertson (5) prefers the term pleural sarcoma, while Saccone and Coblenz (6), in order to avoid the controversy of cell origin, called the condition a pleuroma. Two groups of mesothelioma have been recognized, the localized and the diffuse. Clinical differentiation is important be-cause of the surgical and prognostic implications. Twelve cases of mesothelioma, 5 of the localized and 7 of the diffuse type are herein reported. Case Reports Localized Pleural Mesothelioma Case I: A 24-year-old white woman was ad-mitted on July 24, 1942. In December 1941, a routine chest film revealed a “shadow” in the right lung. The patient remained asymptomatic until, prior to admission, a dry cough developed, with dull pain in the right lower chest posteriorly. The pain was pleuritic in type and lasted three weeks. There was no weight loss, hemoptysis, or fever. A repeat chest film (Fig. 1) showed increase in size of the mass, and surgery was advised. Physical examination revealed dullness and absence of breath sounds beneath the level of the right fifth rib posteriorly. The roentgenogram disclosed a localized area of increased density in the posterior portion of the right lower chest. On thoracotomy the right lower lobe was found to be almost completely replaced by a large firm globular mass, and a lobectomy was performed. There were no enlarged lymph nodes in the mediastinum. Pathologic examination revealed a large ovoid tumor weighing 360 gm. and measuring 11 × 10 × 5 cm. The mass was sharply circumscribed but did not possess a distinct capsule. Histologic examination showed papillary processes consisting of well defined flat cells which resembled endothelial cells and sheets of large polyhedral or ovoid cells. In some places the neoplasm replaced the alveoli and, although mitotic figures were not numerous, the variation in cell morphology suggested malignancy. Follow-up examination twelve years later showed a well patient with no evidence of disease (Fig. 2). Case II: A 71-year old white man was admitted on June 2, 1950, with pain in the right anterior chest wall of one month duration. He had experienced a productive cough, and the sputum was occasionally blood-streaked. Physical examination was unremarkable. The lungs were clear.