Mucinous epithelial neoplasms arising in association with mature teratomas are a heterogeneous group of tumors, but with the exception of a single recent study, their full histologic spectrum, detailed immunophenotype, and association with classic pseudomyxoma peritonei (PMP) have not been fully studied. The morphologic, immunohistochemical, and clinical features of 42 patients with mucinous epithelial tumors arising in association with mature ovarian teratomas were evaluated. The patients' ages ranged from 17 to 66 years (mean, 39 y). Tumor size ranged from 5.5 to greater than 200 cm. Most teratoma-associated mucinous tumors were unilateral, although 1 patient harbored bilateral mucinous tumors in association with bilateral teratomas. In all cases, the teratomatous component consisted of mature elements. Using the 2003 World Health Organization criteria for ovarian intestinal type mucinous neoplasms, 17 (40%) were classified as mucinous cystadenoma, 16 (38%) as intestinal-type mucinous epithelial neoplasm of low malignant potential (IM-LMP), 4 (10%) as intraepithelial carcinoma (IEC), and 5 (12%) as invasive mucinous carcinoma. Mucinous cystadenomas had a varied epithelial lining consisting of lower gastroenteric, gastric foveolar, or müllerian appearance. In contrast, the IM-LMP, IEC, and invasive carcinoma cases had a more uniform lower gastroenteric histology. For mucinous cystadenomas, a cytokeratin (CK) 7+/CK20- phenotype (5/13; 38%) was equally as common as a CK7-/CK20+ phenotype (5/13; 38%), with the remaining cases coexpressing both keratins (CK7+/CK20+: 3/13; 23%). In contrast, IM-LMP, IEC, and invasive adenocarcinomas more frequently had a CK7-/CK20+ phenotype (56%, 50%, and 100%, respectively). A CK7+/CK20-phenotype was rare in these later 3 morphologic groups (6%). Of the 42 total cases, 55% had pseudomyxoma ovarii and 24% had classic PMP (1 cystadenoma, 6 IM-LMP, and 3 invasive carcinomas), whereas 5% had more localized accumulations of peritoneal mucin (both IM-LMP). Pathologic evaluation of the peritoneum in these 12 cases revealed 6 with acellular mucin alone, 3 with low-grade mucinous epithelium (all 3 with ovarian IM-LMP), and 3 with high-grade mucinous carcinomatosis (all 3 with ovarian mucinous adenocarcinoma). No appendiceal lesions were identified. Follow-up was available in 48% of patients (mean, 61 mo). The only adverse outcomes occurred in the 3 patients with ovarian carcinoma and associated peritoneal carcinomatosis. We report that a significant proportion of mucinous tumors associated with mature ovarian teratomas present with clinical PMP, which in most cases is associated with IM-LMP. PMP in this setting may harbor microscopic intra-abdominal low-grade mucinous epithelium that is histologically and immunophenotypically similar to that typically seen in appendiceal-related PMP. Pseudomyxoma ovarii is common in this setting, particularly in tumors with IM-LMP histology, but pseudomyxoma ovarii is not predictive of PMP. Ovarian teratoma-associated benign and IM-LMP mucinous neoplasms with microscopic peritoneal low-grade mucinous epithelium do not seem to be at significant risk for intra-abdominal recurrence, but numbers are few and follow-up is limited. In contrast, teratomas with an invasive carcinomatous component and microscopic peritoneal carcinomatosis follow an aggressive clinical course.