Primary ovarian mucinous carcinomas of the intestinal type are uncommon and earlier reports have included cases diagnosed according to older, less stringent, criteria (which would now be considered borderline tumors) and variable numbers of cases of metastatic adenocarcinoma. This study was conducted to identify all cases of primary mucinous carcinoma of the ovary in a population-based registry, diagnosed according to WHO 2003 criteria, and to characterize their histologic features, immunohistochemical expression profile, and outcome. Thirty-one cases of primary ovarian mucinous carcinoma were included in this study. Immunostaining for 33 markers was performed. Mean age of the patients was 55.4+/-13.5 years. Thirty tumors were stage I or II at presentation. Twenty-six of 31 (83.9%) tumors had expansile stromal invasion, 4 of 31 (12.9%) showed destructive invasion, and 1 of 31 (3.2%) had anaplastic carcinoma in a mural nodule. All cases with destructive invasion showed grade 3 nuclear atypia whereas only 3 of 26 (11.5%) cases with expansile invasion had grade 3 nuclear atypia (P=0.0003). At follow-up, 6 of 26 patients (23.1%) with tumors showing expansile invasion experienced a recurrence, compared with 1 of 4 patients (25%) with destructive invasion and the single patient (100%) with anaplastic carcinoma. There was CK7 positivity in 26 of 31 cases (86.7%), and CK20 and Cdx-2 were each positive in 33.3% of cases. D2-40, calretinin, mesothelin, CA-125, Pax-8, TTF, and WT1 were completely negative in all cases. NHERF1 staining was present in 19 of 26 cases (73%) and its expression was associated with poor prognosis (P=0.05). Our findings support current diagnostic criteria for primary ovarian mucinous carcinoma, that is, the presence of expansile invasion, in the absence of destructive invasion, warrants a diagnosis of carcinoma. A large majority of mucinous carcinomas show only an expansile pattern of invasion and are confined to the pelvis at diagnosis.