Deciduoid mesothelioma is a rare variant of the epithelioid histotype spectrum, resembling decidua of gravid uterus. It is found in the peritoneum of young women, but also in the pleura of elderly people. Histotype plasticity from epithelioid to sarcomatoid mesothelioma may be considered as epithelial-mesenchymal transition (EMT). A full autopsy was performed and mesothelioma infiltrates were analysed by immunohistochemistry. The metastasis of an epithelioid pleural mesothelioma to a hyperplastic polyp of the stomach is presented in this autopsy case. Deciduoid morphology increased during tumour progression and metastasis. The increase in eosinophilic cytoplasm correlated with the upregulation of the intermediate filament vimentin. High expression of vimentin was found in both central and superficial periglandular regions of the polyp. High vimentin expression also can occur in epithelioid rather than sarcomatoid differentiation. Thus, although vimentin is considered to be the major EMT marker, additional pathways must regulate its expression.