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Fifty-seven cases of mesothelioma from the Toronto General Hospital during the period 1965 to 1976 have been reviewed. Pleural effusions or ascites were present in 34 patients. The value of cytologic examination of effusion specimens was assessed, and the criteria for cytologic diagnosis were elucidated. It appears that patients with predominantly fibrous or sarcomatous mesothelioma were not prone to develop pleural effusions or ascites; few tumor cells, if any, from these mesotheliomas were exfoliated into effusions. Cytologic examinations of effusion specimens were positive in 12 of 14 cases of carcinomatous mesothelioma and in three of four cases of undifferentiated mesothelioma. However, only four of seven casee of benign mesothelioma (mostly epithelial type) showed positive results, as did two of four cases of sarcomatous mesothelioma. It appears that cytologic diagnosis of mesothelioma is more useful for the carcinomatous and undifferentiated types. The cytomorphologic features of the various types of mesothelioma are different, and by cytologic examination of effusion specimens, typing of mesothelioma is possible and correlates well with the histologic typing.