The natural history, classification, and nomenclature of ovarian serous tumors of low malignant potential (S-LMP) (serous tumors of borderline malignancy, atypical proliferating tumors) are controversial. To determine long-term outcome for patients with S-LMP and further evaluate whether S-LMP can be stratified into clinically benign and malignant groups, the clinicopathologic features of 276 patients with S-LMP and > or =5 year follow-up were studied. The histology of the ovarian primary, extraovarian implants, and recurrent tumor(s) were characterized using World Health Organization criteria and correlated with FIGO stage and clinical follow-up. After censoring nontumor deaths, overall survival and disease-free survival for the 276 patients was 95% (98% FIGO stage I; 91% FIGO II-IV) and 78% (87% FIGO stage I; 65% FIGO stage II-IV), respectively. Unresectable disease (P < 0.001) and invasive implants (P < 0.001) were associated with decreased survival. When compared with typical S-LMP, S-LMP with micropapillary features were more strongly associated with invasive implants (P < 0.008) and decreased overall survival (P = 0.004), but patient outcome with micropapillary S-LMP was not independent of implant type. Stromal microinvasion in the primary tumor was also correlated with adverse outcome, independent of stage of disease, micropapillary architecture, and implant type (P = 0.03). There was no association between outcome and lymph node status. Transformation to low-grade serous carcinoma occurred in 6.8% of patients at intervals of 7 to 288 months (58% > or = 60 months) and was strongly associated with increased tempo of disease and decreased survival (P < 0.001). S-LMP forms a heterogeneous group, morphologically and clinically distinct from benign serous tumors and serous carcinoma. The majority of S-LMP are clinically benign, but recurrences are not uncommon, and persistent disease as well as deaths occur. Progression to low-grade serous carcinoma is highly predictive of more aggressive disease. Other features associated with recurrent and/or progressive disease include FIGO stage, invasive implants, microinvasion in the primary tumor, and micropapillary architecture. These predictors tend to co-occur, and no single clinical or pathologic feature or combination of features identify all adverse outcomes. The small, but significant risk of progression over time to low-grade serous carcinoma emphasizes the need for prolonged follow-up in patients with S-LMP.