Aromatase cytochrome P-450 was immunohistochemically localised exclusively in the cytoplasm of neoplastic cells of benign and malignant nonfunctional ovarian tumours, whilst (progesterone receptors (PR) and estrogen receptors) (ER) were localised exclusively inn the nuclei of neoplastic cells. Aromatase activity and PR were detected in 68% (17/25) and 32% of the malignant tumours and 82% (22/27) and 67% of the benign tumours, respectively. In postmenopausal tumours, the positivity for PR in malignant tumours was less frequent (p < 0.01) than that in benign tumours. The tumours in which both aromatase and PR were positive, were less frequent (p < 0.05) in malignant than in benign tumours. Aromatase activity was detected in 100% (8/8) of the PR-positive tumours but in only 68% (9/17) of the PR-negative tumours. With postmenopausal malignant tumours, there was a positive correlation between aromatase activity and PR level (r = 0.77, p < 0.001). However, there was no significant difference in the positivity of ER. The serum steroid levels did not correlate with the tumour levels of aromatase activity, PR or ER. These findings suggest that aromatase activity is correlated with PR in ovarian tumours of postmenopausal women. In addition to steroid receptor status, aromatase activity may be a useful factor in ovarian cancer.