Poor outcome in serous borderline tumors (SBT) of the ovary is limited to patients with advanced stage disease. This study was designed to determine whether there are histologic features among advanced-stage SBTs (International Federation of Gynecology and Obstetrics [FIGO] stages II and III) that predict behavior. The 65 cases in the study were divided into three groups: typical SBTs, with noninvasive implants (51 cases), SBTs with invasive implants (three cases), and a recently described tumor, designated micropapillary serous carcinoma (MPSC) (11 cases), a proliferative serous ovarian neoplasm that often lacks destructive infiltrative growth but appears to behave as a low-grade invasive carcinoma. When the tumor lacks infiltrative growth, as it did in the 11 cases in this series, it qualifies as a borderline tumor. After censoring nontumor deaths, the 5- and 10-year actuarial survival rates were 98% for SBTs with noninvasive implants, 33% for SBTs with invasive implants, and 81% at 5 years and 71% at 10 years for MPSCs. The mean follow-up was 100 months. Two (4%) of 51 patients with SBTs with noninvasive implants subsequently developed invasive carcinoma, and one (2%) died of carcinoma. In contrast, two (67%) of three women with SBTs accompanied by invasive implants developed invasive carcinoma, and both died of disease. Finally, of the 11 patients with MPSC, seven (64%), all of whom had invasive implants, developed recurrences of invasive carcinoma and/or died of tumor. MPSCs had significantly higher rates of mortality (p < 0.001) and recurrence as invasive carcinoma (p < .002) than SBTs with noninvasive implants. The recognition that SBTs can be divided into benign and malignant subtypes provides the basis for replacing the borderline category. The benign subgroup is composed of typical SBTs, including those with noninvasive implants for which the term atypical proliferative serous tumor is appropriate. In contrast, tumors displaying a micropapillary growth pattern (MPSC) and SBTs with invasive implants should be classified as carcinomas and treated accordingly.