Approximately 3000 American women are diagnosed with borderline ovarian tumours annually. Common signs and symptoms include abdominal/pelvic pain and a palpable adnexal mass. Pelvic sonography may be helpful, although not specific, in the diagnosis. Serum CA 125 is abnormal in only about 50% of patients. Primary surgery is the principal treatment; it consists of resection of the primary tumour(s) (frequently in the form of fertility-sparing surgery), frozen-section analysis and consideration of comprehensive surgical staging. The role of surgical staging remains unclear; further research is necessary. For patients with stage I disease, surgery alone is the standard. For patients with stage II-IV disease (with non-invasive or invasive peritoneal implants), the role of post-operative therapy remains unclear. Approximately 20-30% of the latter will relapse, frequently after several years. Most so-called recurrences are low-grade carcinomas. Potential predictive or prognostic factors include age, FIGO stage, residual disease and the micropapillary pattern. After fertility-sparing surgery, most patients retain normal reproductive function.