Mucinous carcinomas are reported to comprise 6-25% of ovarian carcinomas (mean 12%), but recent refinements in the interpretation of histologic features of noninvasive and metastatic mucinous carcinomas suggest that this may be an overestimate. Mucinous carcinomas in the ovaries are commonly metastatic, but the proportion of primary versus metastatic mucinous carcinomas in unselected patients is unknown. To evaluate the histologic-type distribution of ovarian surface epithelial carcinomas, a consecutive series of 124 ovarian carcinomas was reviewed using uniform current criteria. Microinvasive and intraepithelial carcinomas and carcinomas arising in germ cell and stromal tumors were excluded. To evaluate the proportions of primary and metastatic tumors among the mucinous carcinomas, 52 consecutive mucinous carcinomas from nonreferral patients were reviewed. Three of 124 primary ovarian carcinomas were pure mucinous carcinomas (2.4%). Among 52 cases of mucinous carcinomas in the ovaries, 40 (77%) were metastatic and 12 (23%) were primary. Among the 12 primary mucinous tumors, three were atypical proliferative mucinous tumors with microinvasion and nine were invasive mucinous carcinomas. Among the 40 metastatic mucinous tumors, 18 (45%) were from the gastrointestinal tract, 8 (20%) were pancreatic, 7 (18%) were gynecologic malignancies (5 cervical, 2 endometrial), 3 (8%) were from the breast, and 4 (10%) were of unknown primary site. A simple rule that classifies all bilateral mucinous carcinomas as metastatic, unilateral mucinous carcinomas <10 cm as metastatic, and unilateral mucinous carcinomas >or=10 cm as primary correctly classified 90% of the neoplasms. This algorithm for distinguishing primary and metastatic mucinous carcinomas in the ovary can be used at the time of intraoperative consultation to guide surgical management. After careful exclusion of noninvasive, microinvasive, and metastatic tumors, pure mucinous adenocarcinoma primary in the ovary appears to be substantially less common than previously reported.