Seromucinous neoplasms are a new category of ovarian epithelial tumor in the revised World Health Organization Classification of Tumours of the Female Reproductive Organs. Borderline variants are well described, but there have been few reports of seromucinous carcinomas. We report the clinicopathologic features in 19 cases of ovarian seromucinous carcinoma in patients aged 16 to 79 years (mean 47). In 16 cases, the neoplasm was unilateral and in 3 cases bilateral. The tumors ranged in size from 1.8 to 18 cm (mean 10.5 cm). The tumors were stage I (n=15), stage II (n=1), and stage III (n=3). The histologic features were highly variable both within and between individual tumors. The majority of neoplasms (12 cases) exhibited a predominant papillary architecture with lesser components of glandular, microglandular, and solid growth. A predominant glandular architecture was present in 6 cases, whereas 1 had a predominantly solid growth. A characteristic feature was an admixture of cell types. Most of the tumors (15 cases) were mainly composed of endocervical-like mucinous cells, whereas in 4 cases there was predominant endometrioid differentiation. Other cell types, present in varying proportions, included hobnail cells, eosinophilic cells, squamous cells, clear cells, and signet-ring cells. An infiltrate of neutrophil polymorphs was a prominent feature in most cases. Most cases also exhibited areas of microglandular architecture with cytoplasmic clearing and intraluminal polymorphs, the features closely resembling cervical microglandular hyperplasia. Areas of stromal hyalinization, adenofibromatous growth, and psammoma bodies were present in a minority of cases. Endometriosis was identified in the same ovary in 10 cases, and in 10 there was a component of seromucinous borderline tumor. Thirteen, 5, and 1 tumor were of grades 1, 2, and 3, respectively (using the FIGO grading system for endometrioid adenocarcinomas of the uterine corpus). A synchronous uterine endometrioid adenocarcinoma was present in 1 case. Immunohistochemically, there was positive staining with CK7 (17/17 cases), estrogen receptor (16/16 cases), progesterone receptor (6/7 cases), CA125 (15/15 cases), PAX8 (8/8 cases), CEA (8/13 cases), CA19.9 (8/9 cases), and WT1 (2/13 cases). CK20 and CDX2 were negative in all cases tested (16 and 14, respectively). p53 showed "wild-type" staining in 4/4 cases, and p16 was focally positive in 5/5 cases. Follow-up information was available in 8 patients. Seven were alive with no evidence of disease (follow-up 3 to 74 mo), whereas 1 patient who initially presented with a stage IIB tumor died of disease at 192 months. Given the characteristic admixture of cell types and the overlapping morphologic features with low-grade serous, mucinous, and endometrioid neoplasms, the most appropriate categorization of seromucinous carcinomas is uncertain, but we believe they are best regarded as a distinct type of ovarian epithelial malignancy and are most similar to endometrioid adenocarcinomas. We recommend grading them in an analogous manner to ovarian endometrioid adenocarcinomas.