Mesothelioma, even of the pleura alone, is a ubiquitous pathological designation. The diffuse variety is associated with an ominous prognosis. The localized mesothelioma generally has been assumed to be benign. A review of tumors from 60 patients with localized pleural mesothelioma seems to justify separation into benign (52 patients) and malignant (8 patients) variants. Histologic criteria are used primarily to make this division. No single clinical feature allowed preoperative predictability concerning benignity or malignancy, although extrathoracic osteoarthropathy was seen only with benign lesions (20%), usually those greater than 7 cm. Follow-up periods of as long as 24 yr revealed recurrent tumor in only 2 of the 58 patients with benign lesions and revealed a survival curve identical to that of the general population. All of the patients with the malignant variety had recurrence or metastasis, and all but one were dead within 2 yr of the initial operation. One patient died of tumor complications after 9 yr. More aggressive local resection is indicated for the malignant variant.