The distinction of malignant mesothelioma from reactive mesothelial proliferation remains to be a major challenge for surgical pathologists. In this study, we investigated whether insulin-like growth factor II messenger ribonucleic acid-binding protein 3 (IMP3), an oncofetal protein, can be used as a biomarker to distinguish between malignant and reactive mesothelial cells. A total of 109 cases (mesothelioma, n=45; reactive mesothelial proliferation, n=64) were examined by immunohistochemistry for IMP3 expression. IMP3 showed strong cytoplasmic staining in 33 of 45 (73%) mesothelioma cases. In contrast, the expression of IMP3 was undetectable in all (64 cases) benign reactive mesothelial proliferations. Among the IMP3-positive mesotheliomas, 27 (82%) exhibited diffuse IMP3 expression. The vast majority of IMP3-positive subtypes of mesotheliomas showed IMP3 expression in >50% of malignant cells, as this diffuse staining pattern occurred in 17 (81%) cases of epithelial, 4 (100%) cases of sarcomatoid, and 6 (75%) cases of mixed types of mesothelioma. In addition, 2 cases, which were initially diagnosed as atypical mesothelial proliferations and later confirmed to be mesotheliomas, showed diffuse IMP3 expression. Our findings suggest that IMP3 is a new positive biomarker for malignant mesothelioma. IMP3 immunohistochemical staining can be used as an adjunct tool in the distinction of malignant mesothelioma from reactive mesothelial proliferations.