Background:   Malignant mesothelioma (MM) is an aggressive tumor which arises from the lining of serous cavities. Distinguishing between begnin and malignant mesothelial proliferation on effusions is a challenge. This study aims to evaluate the diagnostic utility of BAP1 in distinguishing MM from reactive mesothelial proliferation.Material and methods: This is a retrospective study on 61 cases; pleural biopsies (n=36) and pleural cell blocks (n=25). Included were synchronous cytology/ biopsy pair samples. All cases were stained with BAP-1antibody using avidin-biotin complex. BAP1 immunohistochemistry was evaluated using cut off value; negative staining indicate malignancy. Statistical analysis was done using SPSS (version 20), P value (P value Results: In MM cases 61.1% showed BAP1 negative nuclear expression. In reactive mesothelial proliferation cases; 20% showed BAP1 negative expression. In atypical mesothelial proliferation cases, 80% showed BAP1 negative nuclear expression. synchronous cytology/ biopsy pairs (13 cases) showed BAP1 matching results. There was a highly statistically significant correlation between BAP1 expression and the study groups (P-value <0.001). There was statistically significant correlation between BAP-1 expression and histological types of mesothelioma (P-value <0.05) and stage (P-value <0.05). In cell blocks; sensitivity was 80% and specificity was 80% for atypical mesothelial proliferation. In tissue biopsy; sensitivity was 61.1% and Specificity was 80% for mesothelioma.Conclusion: In effusions, negative BAP1 strongly support the diagnosis of malignant mesothelioma, so BAP1 may be included in immunohistochemical panels for malignant mesothelioma cytodiagnosis.