Serous ovarian tumours on malignancy borderline represent 2 to 5% of the epithelial ovarian tumours. They differ from malignant tumours by two particularities, namely: precise histological characteristics, the main one being the absence of stroma invasion, a favourable prognosis. The diagnosis rests on conventional histology in spite of the value of histo-immunoclinical markers or tumoral markers. The treatment is surgical, based on hysterectomy with bilateral adnexectomy, even if a conservative attitude may be considered for the young woman who wishes a pregnancy. Prolonged post-operative surveillance is very important because of possible relapses, whose detection seems to be facilitated by antigen CA 125 assay.