Only rare primary mucinous (goblet cell) carcinoids of the ovary have been reported, and their clinicopathologic features have not been well delineated. The authors studied 17 examples from patients 14 to 74 years of age. The clinical presentations were similar to those of ovarian neoplasms in general. The tumors ranged from 0.8 to 30 cm in diameter. In six cases the tumor was in the wall of a mature cystic teratoma, appearing grossly as solid nodules or areas of thickening in four of them, six tumors were entirely solid, and five were solid associated with other types of cystic tumor. The tumors were divided into three groups on the basis of their microscopic features. Six neoplasms, designated "well differentiated," were composed of small glands, many of which floated in pools of mucin. The glands were lined by goblet cells and columnar cells, some of which were of neuroendocrine type. Three tumors, designated "atypical," were characterized by crowded glands, some of which were confluent, small islands with a cribriform pattern, and scattered microcystic glands. The glands were lined by cuboidal to columnar cells, some of them neuroendocrine, admixed with goblet cells. Eight tumors, designated "carcinoma arising in mucinous carcinoid," contained islands and larger nodules of tumor cells, or closely packed glands, as well as single cells, mainly of the signet ring cell type. Most of the cells were devoid of mucin and were severely atypical with marked mitotic activity. Necrosis was present in all eight tumors. Seven of the eight tumors with a carcinomatous component contained at least minor foci of well-differentiated mucinous carcinoid; the eighth contained only foci of atypical mucinous carcinoid. The neuroendocrine nature of a variable proportion of the cells in all three groups was demonstrated by staining for neuroendocrine markers. The mucinous nature of other cells was confirmed by mucicarmine or Alcian blue stains. The ovary contained an intrinsic component of trabecular and insular carcinoid, and of strumal carcinoid in one case each, an adjacent mature cystic teratoma in six cases, mucinous cystadenocarcinoma in three cases, and borderline mucinous cystic tumor, borderline Brenner tumor, and epidermoid cyst in one case each. Fifteen tumors were stage I, one was stage II, and one was stage III. The last two tumors had a carcinomatous component. Follow-up data were available for 15 patients; 12 were alive and free of tumor 2.3 to 14 years (average, 4.7 years) after the ovarian tumor was excised. One patient, whose tumor had a carcinomatous component, died 3 years postoperatively of unrelated causes. Two patients, both of whom had a carcinomatous component in their tumor, died 9 and 12 months postoperatively. Primary mucinous carcinoids must be distinguished from metastatic mucinous carcinoid tumors from the appendix or elsewhere. Features supporting an ovarian origin are the additional presence in the specimen of teratoma or an ovarian surface epithelial tumor, an absence of blood vessel or lymphatic space invasion, and confinement to a single ovary. Similar features help to distinguish mucinous carcinoids from Krukenberg tumors. Mucinous carcinoids should also be distinguished from strumal carcinoids, which can contain mucinous glands, and insular carcinoid tumors that arise rarely in the wall of a mucinous cystic neoplasm. Although the number of cases in this series is small, the follow-up data suggest that the degree of differentiation, particularly the presence of frank carcinoma, is an important prognostic factor.