In an effort to improve our knowledge of certain marker substances in human ovarian neoplastic diseases (both benign and malignant) 12 patients with ovarian tumors were studied. Ten of these were postmenopausal. Venous blood was drawn simultaneously from both ovarian veins and a cubital vein by direct puncture during laparotomy. In cystic tumors, cyst fluid was also collected. 17β-estradiol and progesterone were analyzed in all samples, as were FSH, LH, and CEA in peripheral venous blood and cyst fluid. Owing to the physiological variation of gonadal steroid levels in premenopausal women, abnormal production of steroids from the tumors could be detected only in postmenopausal women. The study showed that 4 out of 6 postmenopausal women with benign ovarian tumors had abnormally high peripheral blood levels of estradiol and progesterone emanating from the ovaries harboring the tumors. All 4 postmenopausal women with malignant tumors likewise had an abnormally high production of estradiol, and usually also of progesterone. High concentrations of estradiol were seen in tumorous ovarian venous blood in one patient in spite of normal concentrations in peripheral venous blood. Various histological types of tumor could produce similar hormonal profiles. One case out of 6 of benign ovarian tumor and 2 cases out of 6 of malignant ovarian tumor showed elevated levels of CEA in the peripheral blood. One benign serous and 2 benign mucinous cystomas had elevated CEA values in the cyst fluid. This preliminary report demonstrates that 8 out of 10 postmenopausal women with ovarian tumors had an abnormal production of gonadal steroids. The determining of gonadal steroids and other marker substances in ovarian venous blood sampled, i.e. during angio-catheterization, might improve the early detection of ovarian tumors. Relevant studies are in progress.