Localized malignant mesotheliomas are uncommon sharply circumscribed tumors of the serosal membranes with the microscopic appearance of diffuse malignant mesothelioma but without any evidence of diffuse spread. Little is known about their behavior. We report 23 new cases. The mean age at presentation was 63 years, and the sex ratio was approximately 2:1 (male/female). Twenty-one tumors were pleural and 2 were peritoneal. Sixteen tumors reproduced microscopic patterns of diffuse epithelial mesotheliomas, 6 had mixed epithelial and sarcomatous patterns, and 1 was purely sarcomatous. After surgical excision of the tumor, 10 of 21 patients with follow-up data were alive without evidence of disease from 18 months to 11 years after diagnosis. Patients who died had developed local recurrences and metastases, but none had diffuse pleural spread. Localized malignant mesotheliomas should be separated from diffuse malignant mesotheliomas because of their localized presentation, quite different biologic behavior, and far better prognosis.