Three cases of mesotheliomas diagnosed by cytologia examination. Three cases of pericardia! mesotheliomas which were diagnosed by cytologic examinations are presented. One of the cases was a 46 years-old man. The others were women of 23 and 74 years old. Cytologic examination of tumor cells in pericardial fluidb is not used widely in our country. In this report the malignancy criteriae of mesothelial cells were studied and the differential diagnosis with the other malignancies was discussed.