It has been urged recently that the surface tumors of the ovary be classified as mesotheliomas because both of these neoplasms have a common ancestry. It was suggested also that the rare extragonadal peritoneal tumor of a microscopic morphology similar to that of the ovarian tumor be considered as a mesothelioma. In the present report, objections to this classification are offered. Fifteen cases of diffuse and/or localized peritoneal tumors interpreted as papillary carcinoma arising from extraovarian tissue of Müllerian potentiality are described, and distinctions from mesothelioma are pointed out. Reasons for opposing the grouping of ovarian carcinoma or extragonadal papillary carcinoma of the peritoneum with mesothelioma include the need for categorizing the latter separately in order to monitor its association with asbestos exposure and the possibility that biological differences between these tumors may lead to the development of different modes of therapy.