Among the various antigens described as biological markers of ovarian cancer, only one, CA 125, is of value in clinical practice. CA 125 is not specific to epithelial ovarian cancer, but it is associated with this tumour. CA 125 assays are useless in the early detection of tumours in asymptomatic patients, but they are helpful to monitor the clinical course of treated cystadenocarcinoma of the ovary since a rise in CA 125 level may be observed several months before clinical or radiological evidence of ovarian malignancy is obtained. Finally, CA 125 determination is important before second-look laparotomy.