Peripheral serum concentrations of progesterone were measured in 27 postmenopausal women with malignant, borderline, or benign ovarian tumor and in 10 women without ovarian neoplasm. The progesterone levels were significantly higher in all ovarian tumor groups as compared with the controls. There was no difference between malignant and benign tumors. Patients with a tumor volume over 1 000 ml had higher progesterone levels than patients with smaller tumors, in both peripheral and ovarian venous blood. Mucinous ovarian tumors were associated with the highest progesterone levels, compared with other histologic types of ovarian tumor. The results indicate that mucinous ovarian tumors are able to secrete progesterone and that the stage of malignancy has no effect on this hormonal activity.