BACKGROUND Malignant pleural mesothelioma is a rare tumor with a dismal prognosis, usually presenting with recurrent effusions. However, the majority of malignant pleural effusions are due to lung adenocarcinoma (AdC). The distinction between these tumors has considerable therapeutic and medicolegal implications and can be very challenging both histologically and cytologically. Appropriate immunohistochemistry (IHC) is required to support the diagnosis. MicroRNA (miRNA) expression analysis could be a viable diagnostic tool for distinguishing between these tumors. The purpose of the current study was to assess the reliability of miRNAs as diagnostic markers to differentiate epithelioid malignant mesothelioma (MM) from lung AdC. METHODS Bioinformatic analysis of publicly searchable data sets regarding miRNA expression profiling was performed to select the most significant differentially expressed miRNAs. These were analyzed by quantitative polymerase chain reaction on histologic (41 MM cases and 40 lung AdC cases) and cytological (26 MM cases and 27 lung AdC cases) specimens and the diagnostic performances were assessed. RESULTS miR-130a, miR-193a, miR-675, miR-141, miR-205, and miR-375 were found to be the best distinguishing markers. Of these, only miR-130a was significantly overexpressed in MM compared with lung AdC (P =.029 in histologic and P =.014 in cytological samples). miR-130a demonstrated a sensitivity of 77%, a specificity of 67%, a positive predictive value of 69%, a negative predictive value of 75%, and an accuracy of 72% in identifying MM. CONCLUSIONS The diagnostic performances of miR-130a expression analysis and IHC appear to be similar. miR-130a quantification could be used reliably as second-level diagnostic tool to differentiate MM from lung AdC in pleural effusion cytology, mainly in those cases with ambiguous or negative IHC. Further validation is needed. Cancer Cytopathol 2017;125:635-43. © 2017 American Cancer Society.