Various laboratory methods are being used to acquire diagnosis in pleural effusions. However, about 20% of the effusions cannot be diagnosed reliably. Cytogenetic analysis in pleural effusion is not used routinely, although many numerical and/or structural chromosome abnormalities have been observed in malignant pleural effusions. In this study, a total of the 61 pleural effusion samples, 34 malignant which included 19 diffuse pleural malignant mesothelioma, 15 metastatic malignant pleural effusions and 27 benign, were analyzed by direct chromosome analysis method. To the findings obtained in the study, 85.3% (29/34) of the 34 patients with malignant pleural effusion had numerical and/or structural abnormalities, and 3 of them had no mitosis. The patients who had benign pleural effusion indicated no numerical and/or structural abnormalities. We have concluded that if a pleural effusion cannot be reliably differentiated by the usual laboratory methods and especially malignancy is strongly suspected, cytogenetic analysis can be used to differentiate malignant effusions from benign effusions with a small rate of error, and also it can indicate that more invasive diagnostic procedures are necessary.