OBJECTIVE: To identify prognostic factors that may be used to predict an aggressive disease course and poor outcome in patients with epithelial ovarian tumors of low malignant potential (borderline tumors). METHODS: Data on 126 patients with ovarian borderline tumors were analyzed with regard to demographic characteristics, staging, presence of microinvasion, duration of follow-up, recurrence rate, rate of recurrence as invasive disease, mortality rate, preoperative and postoperative CA-125, and treatment. Chi-square and Fisher exact tests were used to evaluate proportions for statistical significance. Disease-free and overall survival was calculated by using the Kaplan-Meier method and log-rank test. RESULTS: Patients were followed for a median of 39.0 months (mean 47.8 months). Seven patients (5.6%) had recurrent disease. Advanced stage disease and microinvasion were associated with significantly higher recurrence and mortality rates than were stage I/II disease and borderline tumors without microinvasion, respectively. Two of 13 (15%, 95% CI 8.7, 21.3) patients with microinvasion died of recurrent invasive cancer, whereas only 1 out of 113 patients without microinvasion died of recurrent borderline tumor (OR 20.4, 95% CI 1.2, 239). All 3 patients with an aggressive disease course and poor outcome had increasing CA 125 levels at the time of recurrence. CONCLUSION: Certain patients with microinvasion may be at higher risk for recurrence as invasive disease and may require different treatment strategies. CA 125 monitoring may have a role in early detection of recurrence in patients with aggressive disease.