Mesothelial cells morphological alterations in serous effusion samples are a challenging problem which may mimic malignancy in many circumstances. The aim of this study is to clarify these changes and do a clinicopathological statistical study on serous effusion samples received in Cytology unit, Pathology department, Faculty of medicine, Cairo University, during the last five years (2006-2010).This work included 1635 cases of serous effusions samples, consisted of 65 cases of pericardial effusion, 673 cases of pleural effusion and 897 cases of ascitic effusion. In all pericardial effusion cases mesothelial cells were cytologically benign with no malignant mesothelioma cases. In pleural effusion cases the mesothelial cells were cytologically benign in (97.6%) and (2.4%) of which diagnosed cytologically as malignant mesothelioma. While; in ascitic effusion cases the mesothelial cells were cytologically benign in (99.9%), with one case diagnosed as malignant mesothelioma. This work may be put as a nucleus and linked with other registries as regards the serous effusions in different governorates in Egypt.