Measurements of estradiol production rates before and after castration in women with breast cancer indicated that postmenopausal ovaries made no contribution to estrogen production. A review of previous reports of estrogen measurements and of beneficial responses after ovariectomy in postmenopausal women gave little evidence to support the policy of therapeutic castration in such patients with breast cancer. Therapeutic oophorectomy without adrenalectomy is probably not justified in postmenopausal women with breast cancer.