Deciduoid mesothelioma was first described in young females and in the peritoneum, which led to the suggestion that deciduoid mesothelioma was a distinct subtype with specific clinical and pathologic features. Later reports, however, have shown that this type of mesothelioma may also occur in elderly people and in the pleura. Cases reported in the literature so far are limited, and the disease is not well defined. The authors report the histologic, immunohistochemical, ultrastructural, and clinical findings of 5 cases of deciduoid mesothelioma, and review the literature reports. The results demonstrate that the presence of numerous cytoplasmic intermediate filaments, either dispersed or bundled, appear to be the likely ultrastructural basis for the deciduoid histologic appearance. Twenty-one cases of deciduoid mesothelioma were identified in the literature. Analyses of these 21 cases and the authors' 5 cases showed an age range of 13-78 years (median 53 years) and a slight female predominance (female to male ratio of 1.4:1). Fourteen of 26 cases (54%) occurred in the peritoneum. Seven of 20 patients (35%) had a documented history of asbestos exposure. Fifteen of 20 patients died, with a mean survival time of 7.33 months (range 1-21 months). Five of 20 patients were alive at a follow-up time of 8 months to 5 years. These findings suggest that the so-called deciduoid mesothelioma has some clinical and pathologic features that are dissimilar to mesothelioma in general. Whether it truly represents a pathogenetically distinct variant or merely an expansion of the morphologic spectrum awaits further studies.