The cytohistology in 82 cases diagnosed as malignant peritoneal mesothelioma was correlated with available clinical and gross pathologic information. The cases were then evaluated as to ceratainty of diagnosis. The material had come from a large number of sources, most of it having been traced by a history of occupational exposure to asbestos. A relatively short interval of significant symptoms, with already existent diffuse peritoneal involvement and ascites, and an average survival time of less than a year characterized the group. The microscopic morphology formed a spectrum from highly characteristic, pure epithelial and mixed epithelial and sarcomatoid types, through nonspecific although relatively differentiated appearances, to pleomorphic analplatic proliferations. Local invasion and metastasis were common but much more limited than with tumors of other histogenesis showing comparable serous membrane involvement. Autopsy was of considerable exclusionary value although not in itself always determinative, and mucopolysaccharide histochemistry was occasionally decisive in diagnosis. Because of the microscopic versatlity of mesothelioma and the clinical and gross morphologic overlap with other neoplasms, all available data must be taken into consideration in arriving at a diagnosis. We believe that the dgree of certainty of diagnosis should be indicated by a succinct but reasonably explicit terminology.