Ovarian cancer is the most common fatal malignancy of the female reproductive tract and frequently presents at an advanced stage. There is, thus, a great need for more sensitive and accurate methods of diagnosis, including better tumor markers. A rapidly emerging technique is the application of assays for the inhibin peptide family in patients with various forms of ovarian cancer. Currently, in assays that detect molecules containing the inhibin a subunit, more than 80% of postmenopausal patients with mucinous epithelial ovarian tumors, and virtually all with granulosa- cell tumors, have elevated inhibin levels. The detection of activin in some tumor tissues suggests that this peptide may also prove to be of interest once satisfactory and specific assays for the circulating peptide are available. It is likely that significant progress will be made in this field within the next 5 years.