Recent studies have redefined pseudomyxoma peritonei (PMP) as a specific clinicopathologic syndrome in which mucinous ascites is accompanied by peritoneal lesions characterized by bland to low-grade adenomatous mucinous epithelium intimately associated with pools of extracellular mucin and fibrosis, diagnosed pathologically as disseminated peritoneal adenomucinosis (DPAM). Most recent studies support an appendiceal rather than ovarian origin for virtually all cases of PMP/DPAM in women. Peritoneal mucinous tumors with the histologic features of carcinoma (peritoneal mucinous carcinomatosis, PMCA) are also rarely ovarian in origin and are distinguished from DPAM, even though they may produce abundant mucin, because PMCA and DPAM are pathologically and prognostically distinct. We report three cases of PMP (mucinous ascites) associated with ruptured mucinous tumors arising in ovarian mature cystic teratomas. Two tumors contained bland to low-grade adenomatous mucinous epithelium associated with dissecting mucin, identical to the mucinous tumors that secondarily involve the ovaries and peritoneum in PMP/DPAM derived from ruptured appendiceal mucinous adenomas. The third was composed of both low-grade adenomatous tumor and areas of mucinous carcinoma. In all cases the appendices were microscopically normal. The mucinous ascites associated with the low-grade tumors contained only a few fragments of detached bland mucinous epithelium in one and none in the other. The mucinous ascites associated with the higher-grade tumor contained one fragment of atypical mucinous epithelium. All three mucinous tumors were cytokeratin 20-positive and cytokeratin 7-negative, consistent with a lower gastrointestinal tract-type rather than primary ovarian-type mucinous tumor immunophenotype. Mucinous tumors arising in ovarian mature cystic teratomas are morphologically and immunohistochemically consistent with gastrointestinal tract-type mucinous tumors, which likely arise from gut elements of the teratoma. Rupture can on rare occasions produce mucinous ascites containing very scant mucinous epithelium, but additional follow-up will be required to determine whether these ovarian tumors ever lead to recurrent disease accompanied by the characteristic peritoneal lesions of DPAM or PMCA. Such tumors probably represent the only cases of ovarian origin of PMP.