Some mesotheliomas are composed of bland spindle cells with a haphazard growth pattern and separated by abundant collagenous stroma. They are termed desmoplastic mesotheliomas. Most of them are sarcomatoid type, although desmoplastic features may occasionally be seen with epithelioid mesotheliomas as well. Desmoplastic mesotheliomas are quite rare in the peritoneal cavity. When involving pleura, they need to be distinguished from fibrous pleurisy. Features favoring malignancy include: infiltrating growth pattern, bland necrosis, frankly sarcomatoid areas, and evidence of metastases. Cytokeratin immunostain is usually positive in the tumor cells and can be quite helpful in delineating the infiltrative nature of the process.