Malignant mesothelioma of the pleura most commonly occurs in persons with a heavy occupational exposure to asbestos. Some patients, however, have no such history of exposure. Clinical features include initial complaints of nonpleuritic chest pain and dyspnea. The most frequent roentgenographic finding is a unilateral pleural effusion. Thrombocytosis and elevated erythrocyte sedimentation rates are common. Pleural effusions are typically exudates, are often hemorrhagic, and are usually insufficient for diagnosing mesothelioma based on cytology alone. Even pleural biopsy may not produce enough tissue to enable the pathologist to make a firm diagnosis. Thoracotomy and open biopsy will confirm the diagnosis in most cases. Pathologic distinction from metastatic adenocarcinoma may be difficult even after the use of special stains and electron microscopy. Clinical deterioration is primarily attributable to local spread of tumor. Several factors seem to predict prolonged survival: (1) epithelial histologic subtype, (2) performance score, (3) age of the patient at the time of diagnosis, and (4) absence of chest pain. Surgical treatment, chemotherapy, and irradiation, alone or in combination, have been used for malignant mesothelioma. Except for the palliative effect of irradiation, most treatment protocols have not altered the dismal median survival of approximately 11 months seen in untreated patients with malignant mesothelioma.