For many years the clinical syndrome of PMP has been enigmatic. Based on recent studies reevaluating the condition, tumors previously designated PMP can now be viewed as two pathologically and prognostically distinct disease processes. Disseminated peritoneal adenomucinosis is characterized by copious mucinous ascites (the classical clinical syndrome of PMP) and histologically bland peritoneal mucinous tumors. The condition can be attributed to a ruptured appendiceal mucinous adenoma in the vast majority of cases. It has an indolent clinical course when surgically treated but may recur over months to years. Peritoneal mucinous carcinomatosis is characterized by abundant peritoneal mucinous tumor, similar to the clinical presentation of adenomucinosis. However, microscopically, the peritoneal tumors have the architectural and cytologic features of carcinoma, are derived from gastrointestinal mucinous adenocarcinomas, and are associated with a significantly worse prognosis than cases of adenomucinosis. A third group of tumors displays intermediate or discordant histologic features but manifests a clinical course very similar to cases of pure peritoneal carcinomatosis. Women often have concomitant ovarian mucinous tumors that suggest primary ovarian neoplasia. Morphologic, immunohistochemical, and molecular studies support the interpretation that the ovarian tumors are secondary and that adenomucinosis is of appendiceal origin in women as well as men. The recognition that the ovarian tumors in nearly all of the cases of DPAM and mucinous carcinomatosis are secondary seriously calls into question the existence of a borderline group of ovarian mucinous tumors. Therefore, primary ovarian mucinous tumors should be classified as either benign or malignant. Tumors exhibiting the features currently interpreted as borderline should be included in the benign group and designated atypical proliferative mucinous tumors.