The nomenclature, definition and clinicopathological features of ovarian serous and mucinous tumours of borderline malignancy are reviewed. The reasons are considered for the proposal that serous neoplasms of borderline type should be subdivided into fully benign forms and low-grade adenocarcinomas. Because even the apparently benign forms can recur and can eventually give rise to a frank adenocarcinoma, it is suggested that they should still be considered as a distinct entity. Enteric type borderline mucinous tumours localized to the ovary do appear to be benign, but those associated with pseudomyxoma peritonei are probably metastases from a gastrointestinal carcinoma.