We have investigated the frequency of persistence or recurrence of tumor in 35 patients with ovarian serous borderline tumors treated by unilateral cystectomy, bilateral cystectomy, or unilateral cystectomy with contralateral oophorectomy or salpingo-oophorectomy. Nineteen of the patients had stage Ia, ten had stage Ib, four had stage Ic, and two had stage III tumors. Tumor persisted or recurred only in the ovary that had been subjected to cystectomy in two (6%) of the 33 patients with stage I tumors, in both the ipsilateral and contralateral ovary in one patient (3%), and in the contralateral ovary only in one patient (3%). The frequency of ipsilateral persistence or recurrence of tumor per ovary subjected to cystectomy in this group of patients was 8%. Both patients with stage III tumors had persistence or recurrence of their tumors. Involvement of the resection margin of the cystectomy specimen and the removal of more than one cyst from an ovary were almost always associated with persistence or recurrence of tumor. Follow-up of the 35 patients showed that despite the occasional complication of persistence or recurrence of tumor, all were alive without evidence of disease 3-18 years after the initial operation, with an average follow-up of 7.5 and a median follow-up of 6.5 years.