Pseudomyxoma peritonei (PMP) is a poorly understood condition characterized by mucinous ascites and mucinous implants diffusely involving the peritoneal surfaces. There is considerable debate regarding the definition, pathology, site of origin, and prognosis of PMP. We analyzed the clinicopathologic features of 109 cases of multifocal peritoneal mucinous tumors to develop a pathologic definition of cases characterized by the clinical condition PMP. Cases were separated into two diagnostic categories: disseminated peritoneal adenomucinosis (DPAM) and peritoneal mucinous carcinomatosis (PMCA). Cases classified as DPAM were characterized by peritoneal lesions composed of abundant extracellular mucin containing scant simple to focally proliferative mucinous epithelium with little cytologic atypia or mitotic activity, with or without an associated appendiceal mucinous adenoma. Cases classified as PMCA were characterized by peritoneal lesions composed of more abundant mucinous epithelium with the architectural and cytologic features of carcinoma, with or without an associated primary mucinous adenocarcinoma. Sixty-five of the 109 cases (59.6%) were classified as DPAM consistent with origin from an appendiceal mucinous adenoma. Thirty-seven of the 65 cases (56.9%) had a documented appendiceal mucinous adenoma. Thirty cases (27.5%) were classified as PMCA consistent with origin from an appendiceal or intestinal mucinous adenocarcinoma. Fourteen cases (12.8%) were classified as PMCA with features intermediate between DPAM and PMCA or with discordant features based on the finding of at least focal areas of carcinoma in the peritoneal lesions, whether or not the primary site demonstrated carcinoma. The cases with intermediate features were derived from well-differentiated appendiceal or intestinal mucinous adenocarcinomas and had peritoneal lesions displaying features of DPAM as well as focal areas of mucinous carcinoma. The cases with discordant features were derived from atypical appendiceal adenomas with little or no histologic evidence of a transition from adenoma to carcinoma and had peritoneal lesions uniformly composed of mucinous carcinoma. There was a statistically significant difference in survival between cases classified as DPAM, those classified as PMCA with intermediate or discordant features, and those classified as PMCA (p < 0.0001). The age-adjusted 5-year survival rates were 84% for patients with DPAM, 37.6% for patients with PMCA with intermediate or discordant features, and 6.7% for patients with PMCA. The term DPAM should be used to diagnose the histologically benign peritoneal lesions associated with ruptured appendiceal mucinous adenomas and those that are pathologically identical but lack a demonstrable appendiceal adenoma. Cases with the pathologic features of adenocarcinoma should be designated PMCA because they have recognizably different pathologic features and a significantly worse prognosis.