Peritoneal implants of mucin and columnar epithelium with variable amounts of free peritoneal mucin complicated 11 mucinous ovarian neoplasms. Slides of the primary ovarian tumors were reviewed in eight cases. Although all contained areas of mucinous cystadenomatous tumor of low malignant potential, the neoplasms displayed additional features not well recognized as manifestations of invasive disease in the ovary. Irregular pools of mucin were present in the ovarian stroma and contained at least rare fragments of columnar epithelium in most cases. The epithelium was often more plentiful in the peritoneal mucin. Cytologic atypia was minimal in all except one case. Goblet cells were present in both the ovarian tumor epithelium and the peritoneal implants in most cases and may have a role in dissemination of the tumor. We propose the term "ovarian carcinoma with extracellular mucin production" for this form of mucinous carcinoma; this term is both descriptive of its histologic appearance and indicative of its malignant behavior. Eight of the 11 patients died of their disease from 8 months to 13.5 years after diagnosis. One additional patient died of chemotherapy-related leukemia. The mean survival was 4.2 years. The short-term prognosis is better than that for mucinous carcinoma in general, but most patients eventually die of their disease.