From 1974 to 1980, 69 patients with diffuse malignant mesothelioma were prospectively evaluated. The initial site of involvement was the pleura in 57 patients and the peritoneum in 12. Previous asbestos exposure was found in 53 patients (77%), with a shorter period of latency for peritoneal (mean, 28 years) than for pleural mesothelioma (mean, 35 years). Other associated exposure or diseases included talc, mica, familial Mediterranean fever, and diffuse lymphocytic lymphoma (one patient each). Thrombocytosis was common, as were thromboembolic episodes. Survival was significantly better for patients with an epithelial subtype, with pleural versus peritoneal mesothelioma, and for those under 65 years of age. Surgery was never curative, but its extent was correlated with survival and earlier diagnosis. Results of chemotherapy with doxorubicin and 5-azacytidine yielded a somewhat better survival rate than a combined program with doxorubicin and radiotherapy. Survival after chemotherapy was correlated with performance status, response to chemotherapy, and extent of previous surgery.