AbstractFrom 1977 to 1986, 39 cases at our institution were diagnosed as “consistent with mesothelioma” by percutaneous fine‐needle aspiration biopsy. All were verified clinically andlor histologically, with no false‐positive results. All mesotheliomas except four tumors of peritoneal origin originated from the pleura. On the basis of their cytomorphologic features in correlation with histopathology, the tumors were classified into four types: benign, carcinomatous, sarcomatous, and undifferentiated. Of these 39 cases of mesothelioma diagnosed by aspiration biopsy, nine were benign type; 14, carcinomatous type; 13, sarcomatous type; and three, undifferentiated type. The cytomorphologic features and patterns of different types of mesotheliomas observed in aspirate preparations are presented, and the differential diagnosis is discussed. It appears feasible to investigate pleural‐ or peritoneal‐based lesions with fine‐needle aspiration biopsy, particularly in patients without effusions or with negative results in effusion examinations. It is also possible to make a definitive diagnosis of mesothelioma on the basis of cytologic findings coupled with special and immunoperoxidase staining in combination with roentgenographic findings and cli