One-hundred-twenty-three cases of malignant pleural mesothelioma were reviewed. Exposure to asbestos or to other industrial dusts or chemicals was an important etiologic factor with 24% of patients relating such a history. A history of prior irradiation or previous lung disease was also occasionally noted. Diagnosis was most often made by exploratory thoracotomy, with pleural biopsy or cytology rarely helpful. Except for nine patients, tumor was confined to the chest at the time of diagnosis, but in 33 of the remaining 114 patients, spread to the abdomen or distant metastasis was seen during the course of disease. Surgery and radiotherapy were ineffective in preventing local recurrence. There were only three major responses to chemotherapy in 111 trials. Median survival was 12 months, and only seven patients (5.6%) lived more than five years. Patients with epithelial mesothelioma and Stage I disease had the most favorable prognosis.