Four cases of pleural mesothelioma were treated surgically. The tumors from all cases were microscopicaly malignant, although only in one case the tumor was found to be diffuse in growth. The diagnosis made before operation were exudative pleurisy, empyema or lung cancer with no tumor cells found in examination of pleural fluid, sputum or the specimen of pleuraI biopsy. In two cases only the tumors were resected, and in other two cases pneumonectomy and pleuropneumonectomy were performed. Irradiations added in two cases postoperatively were found not to be beneficial. Postoperative recurrence of tumor growth were found in three cases within two months after surgery, and in one case no evidence of recurrence was noted four and a half months after resection of the tumor.[KTCS 1969;1:49-54]