OBJECTIVES: To examine outcomes in women treated with conservative surgery for borderline ovarian tumor in a large population-based cohort with long-term follow-up. METHODS: Women treated by conservative surgery for borderline tumor of the ovary from 1982-2004 within a large HMO setting were identified using electronic and tumor registry data. Chart review was performed when electronic data were incomplete. The indications for and outcomes from any subsequent gynecologic surgery and the risk of recurrent ovarian borderline and malignant tumor were determined. Risk factors for recurrence were analyzed using multivariate regression. RESULTS: Among one hundred and ninety-three patients identified, the average age was 33 (12-95), with 97% having apparent Stage I disease. Patients were followed with remaining ovarian tissue in situ for a mean of 6.9 years, with 59 women having 10 or more years of such observation. There were 21 recurrences with borderline tumor (11%) with a median time to first recurrence of 4.7 years; women treated by cystectomy recurred three times more often compared to women treated by oophorectomy (23% versus 7%). Two patients (1%) recurred with malignant disease involving remaining ovarian tissue, both within the first 3 years after surgery, with one death due to recurrence. During long-term follow-up, 19% of patients eventually underwent complete removal of ovarian tissue: in 8%, the surgery was prophylactic, in 5%, surgery was done for benign pathology, and in 6% for recurrent disease. CONCLUSIONS: In this population-based HMO setting, 11% of women treated with conservative surgery for borderline tumor recurred; however, half of these recurrences were successfully managed by repeat conservative surgery, with only 6% of women overall needing eventual complete removal of ovaries for recurrent disease. Patients treated by cystectomy were three times more likely to recur than those treated by oophorectomy. Malignant recurrences were rare, and while borderline recurrences often occurred more than 3 years after initial surgery, late malignant recurrences were not observed. These favorable long-term outcomes provide support for conservative surgery for these women.