Ovarian endocervical-type (müllerian) mucinous tumors and tumors composed of a mixture of endocervical-type mucinous, serous, endometrioid, squamous, and indifferent cells with abundant eosinophilic cytoplasm reported to date have been primarily limited to borderline and microinvasive types, with only one report of a disease-related death. The clinicopathologic features of 54 endocervical-type and mixed cell-type mucinous tumors, defined as tumors with papillary architecture resembling serous tumors but containing endocervical-type mucinous epithelium, were evaluated. Thirty-four tumors (64%) were classified as atypical proliferative (borderline) tumors based on the absence of stromal invasion and the absence of micropapillary architecture measuring >5 mm. Five tumors (9%) qualified as intraepithelial carcinoma based on the presence of marked cytologic atypia or a complex cribriform growth pattern involving the epithelium covering the surface of papillae or lining cystic spaces. Eight tumors (15%) with stromal invasion ≤5 mm were classified as microinvasive carcinoma. Seven tumors (13%) with either stromal invasion (five tumors) or micropapillary architecture measuring >5 mm (two tumors) were classified as carcinoma. Sixteen tumors (30%) were bilateral, and endosalpingiosis was identified in 41% of cases. Serous-type differentiation was present in all cases. Of the 29 patients with atypical proliferative tumors, intraepithelial carcinomas, and microinvasive carcinomas for whom follow-up was available, there were no disease-related deaths. In contrast, of the seven patients whose tumors had either stromal invasion or micropapillary architecture >5 mm, two stage III patients died of disease (one with frank invasion and one with a micropapillary tumor that lacked stromal invasion). One other stage III patient with a noninvasive micropapillary carcinoma was alive with disease at 84 months. The remaining four patients (three stage I and one stage III) were alive with no evidence of disease. In summary, most endocervical-type atypical proliferative tumors are stage I and benign. The presence of either intraepithelial carcinoma or microinvasion has no adverse effect on behavior. Rare endocervical-type mucinous tumors demonstrate histologically malignant features and aggressive behavior that warrant designation as carcinoma. As with serous tumors, micropapillary architecture without frank invasion in endocervical-type mucinous tumors is associated with disease recurrence and death when presenting as advanced-stage disease. All the tumors in this study were composed of a heterogeneous population of cells, consisting mainly of serous (ciliated) and endocervical-type mucinous cells. In addition, they all contained endometrioid-type cells, hobnail cells, and indifferent cells with abundant eosinophilic cytoplasm to a varying degree. Accordingly, it appears that tumors that feature endocervical-type mucinous cells are rarely if ever pure but almost invariably of mixed cell type. Despite containing mucinous epithelium, the papillary architecture, serous-type differentiation, association with endosalpingiosis, frequent bilaterality, size, and clinical behavior of endocervical-type mucinous tumors closely resemble serous tumors. We therefore recommend the term “seromucinous” for these tumors, which acknowledges both their serous and mucinous features.