Two cases of multicystic peritoneal mesothelioma (MPM) are reported. Ultrastructural and immunohistochemical techniques confirmed the mesothelial nature of the lesion. The biologic and clinical behaviour, pathogenesis and differential diagnoses of this rare pathology are discussed. Although regarded as a neoplasm, many analogies seem to link MPM to fibromatoses and other non-neoplastic lesions, suggesting a reactive hyperplastic process. The relationships between mesothelium and the secondary Müllerian system, to date not fully investigated, are stressed and a classification of the coelomatic reactive and neoplastic processes, both metaplastic (müllerian metaplasia) and non-metaplastic, is suggested.