With the exception of benign cystadenomas, mucinous ovarian tumors are rare and heterogeneous neoplasms. They have been classified as either borderline tumors or carcinomas for almost 30 years. Subsequently, the borderline tumors have been subclassified into endocervical-like and intestinal types. The diagnostic criteria for distinguishing borderline tumors of the intestinal type from mucinous carcinomas have varied, making difficult the interpretation of prognostic information. More recently, a further subdivision of the former tumors into forms with only epithelial atypia and variants with focal intraepithelial carcinoma has been proposed. Consequently, in this study of 41 mucinous borderline tumors of intestinal type and 34 mucinous carcinomas, the former were also subdivided into 30 cases with mild to moderate atypia only and 11 with areas of intraepithelial carcinoma. All 30 purely borderline tumors were stage I tumors, and all 15 with follow-up information (including one case with microinvasion) were clinically benign. All 11 mucinous borderline tumors that had foci of intraepithelial carcinoma were also stage I neoplasms, and none of the eight patients with follow-up data (including one with microinvasive carcinoma) recurred. Thirty-four invasive carcinomas were subclassified into 15 expansile and 19 infiltrative subtypes. All 15 carcinomas with only expansile invasion were stage I; none of the 11 with follow-up data recurred. Three of nine patients with stage I infiltrative carcinomas with follow-up information had a fatal recurrence. Eight of the remaining 10 infiltrative carcinomas had extended beyond the ovary at the time of diagnosis (stages II and III); of the six patients with follow-up data, four died of tumor and two were alive with disease. In stage I carcinomas nuclear grade and tumor rupture correlated with unfavorable prognosis, but less than infiltrative invasion. However, all three fatal tumors were infiltrative carcinomas that had ruptured, and two contained grade 3 malignant nuclei. Combination of infiltrative invasion, high nuclear grade, and tumor rupture is a strong predictor of recurrence for stage I mucinous ovarian tumors. Among the 19 infiltrative tumors, 13 contained foci of anaplastic carcinoma. Of the seven patients with stage I tumors and follow-up data, only one patient whose tumor had ruptured intraoperatively had a fatal recurrence. The presence of anaplastic components in stage Ia (intact) carcinomas did not have an adverse effect in their outcome, even when the undifferentiated carcinomatous elements appeared in the form of mural nodules.