Background/aim: Micronucleus (MN) frequency is used as a biomarker of chromosomal damage, genome instability, and cancer risk. e aim of this study was to evaluate the diagnostic usefulness of MN frequency to di erentiate between malignant and benign pleural e usion samples. Materials and methods: Retrospectively, 78 pleural uid cytology samples (including 20 cases of benign reactive mesothelial cells, 22 cases of suspicious cytology, and 36 cases of malignant cytology) were examined. e number of micronucleated cells in 1000 well- preserved cells was counted. Statistical tests were performed to compare the study groups. Recover operating characteristics (ROC) curve analysis was performed to suggest a cut-o value for predicting malignant behavior. Results: We evaluated a total of 78 cases of pleural e usion cytology. e number of micronucleated cells was signi cantly higher in cases with malignant outcome compared to cases with benign outcome. We observed that malignant samples had more micronucleated cells than suspicious ones, and suspicious cases had more micronucleated cells than reactive ones. ere was a signi cant di erence among all study groups. In addition, the frequency of MN-containing cells in suspicious cases correlates well with their outcomes. Conclusion: e results of this study reveal that there is an absolute, consistent, and proportional relationship between MN counts and malignancy in cytological samples of pleural e usions. MN scoring may be a helpful diagnostic tool for distinguishing malignant e usions from benign ones, and may be used as an adjunct tool to predict malignant behavior in challenging cases.