To contribution the help of flow cytometry in the diagnosis of mesothelioma in pleural effusions, we studied the aneuploid frequency in paraffin-embedded pleural mesotheliomas. The low aneuploidy rate (31%) explains the moderate yield of mesothelioma diagnosis in pleural effusions. Because of the possibility of false aneuploidies, flow cytometry cannot replace of a cytologic or histopathologic evidence.