Mucinous ovarian neoplasms other than cystadenomas and adenofibromas have been classified as either borderline tumors or carcinomas for many years. Borderline tumors have been subdivided more recently into endocervical-like (mullerian) and intestinal forms. Such a distinction is rarely made in the mucinous carcinoma category. We did not encounter a pure endocervical-like carcinoma in the present series. Criteria for distinguishing an intestinal-type mucinous borderline tumor from a mucinous carcinoma have been controversial. In this study of 164 mucinous borderline tumors of intestinal type and 32 mucinous carcinomas, the former were further subdivided into 74 cases with epithelial atypia only and 90 with focal intraepithelial carcinoma. Of the 67 stage I tumors in the borderline (with atypia) category, all 49 with follow-up data were clinically benign; in the seven cases that had been designated stage III, the intraoperative appearance was that of "pseudomyxoma peritonei," which was fatal in four cases. Most of these tumors, however, were probably metastatic to the ovary rather than truly primary borderline tumors, although failure to examine the appendix in six cases compromised their interpretation. All 90 mucinous borderline tumors that had foci of intraepithelial carcinoma were recorded as stage I, but two of the 69 patients with follow-up data (3%) had fatal recurrences. Both of these tumors were incompletely staged, however, and one had ruptured intraoperatively. Thirty-two invasive carcinomas were subdivided into 12 expansile and 20 infiltrative subtypes; within the latter category seven tumors were only microinvasive. All 12 carcinomas with only expansile invasion were stage I; none of the 10 with follow-up data recurred. All seven microinvasive infiltrative carcinomas were stage I; none of the five with follow-up data recurred. One of five patients with stage I infiltrative carcinomas that were more than microinvasive and were adequately followed had a fatal recurrence, but staging had been incomplete in that case. Seven of the remaining eight infiltrative carcinomas were higher than stage I: five of the six (83%) with follow-up data persisted or recurred and were fatal. Considering all stages, increasing tumor grade in the carcinoma category correlated with an unfavorable outcome. However, grade did not influence prognosis in stage I carcinomas. Among 13 stage I cases in all categories with either preoperative or intraoperative tumor rupture and follow-up data, one recurred, a tumor in the borderline with intraepithelial carcinoma category. "Pseudomyxoma peritonei" is an ill-defined term and should not be used as a pathologic diagnosis. The presence of mucin in the abdominal cavity requires careful histologic evaluation to characterize it for prognostic purposes. Adequate and sometimes extensive sampling of mucinous ovarian tumors, the appendix and the peritoneum in cases of "pseudomyxoma peritonei" is necessary to achieve an accurate diagnosis and prognosis.