Cases (57) of mesothelioma were 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. 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 3 of 4 cases of undifferentiated mesothelioma. Only 4 of 7 cases of benign mesothelioma (mostly epithelial type) showed positive results, as did 2 of 4 cases of sarcomatous mesothelioma. 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.