Twenty-seven cases of desmoplastic diffuse malignant mesothelioma (26 pleural, one peritoneal) are described. In 19 cases the tumor cell type was sarcomatous and in six others it was biphasic (malignant elements of both epithelial and mesenchymal aspect). There were only two cases where the tumor cell type was purely epithelial. The clinical course was often rapid; the mean survival period in 11 cases of purely sarcomatous type was 6.18 months. Only one case of purely sarcomatous type lived for more than 1 year as opposed to four of eight cases with an epithelial component. Metastases occurred more frequently in desmoplastic (60.1%) than in nondesmoplastic diffuse mesothelioma (42.5%). The tumor cell type (epithelial, mesenchymal) accompanying desmoplastic mesotheliomas and not the extent of desmoplasia determines their behavior. Desmoplasia in diffuse mesotheliomas is often the result of tumor cells assuming the functional capacity of fibroblasts and has frequently been confused with reactive fibrosis. Cytologic abnormalities, tissue infiltration, and foci of necrosis indicate the neoplastic nature of the process in most instances.