Proliferating papillary serous cystadenomas that seldom occur are classified into the group of ovarian tumours of low malignant potential (borderline tumours). The present study includes 33 cases observed in our clinic since 1955. Even though the five-year survival rate being 88% was substantially higher than that for ovarian carcinomas, the prognosis of individual cases cannot be reliably predicted because of the uncertainty as to the biological behaviour in both clinical and histological terms. Consequently, the trend to predominantly radical surgery appears to be a justified approach. Whenever it is desirable to maintain fertility in younger women, conservative surgery would require a unilateral tumour without a ruptured capsule and normal histological findings to be obtained from wedge excision in the opposite ovary.