The presence of free fluid in the pleural space can be a sign of many pathological processes. Some of them may be of inflammatory or hemodynamic origin but others are caused by a malignant neoplasm spreading either in the pleural cavity or from distant site. The importance of establishing the origin of the cells in the pleural fliud lies not only in the fact that a correct diagnosis leads to a correct therapy, but it also is essential in the staging of a malignant tumor. The aims of this study were to evaluate the frequency of various types of pleural effusion and to discuss the value of reactive atypia of mesothelial cells in longstanding effusions. The introduction of a third cathegory of pleural effusion called reactive, would be helpful to the general pathologists to avoid making false-positive diagnosis in the presence of atypical reactive mesothelial cells or missing the diagnosis of a malignant effusion in case when malignant cells are scarse or look like an activated mesothelial cell. In this study, a total of 157 patients from the thoracic department of the General Hospital in Kruševac were examined. Of this number, 33 patients or 21% had the so-called reactive type of pleural effusion. In the everyday practice, one in five patients could be expected to have reactive mesothelial atypia, which may be quite confusing in inexperienced hands.

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