Background: Cytotoxic drugs suppress ovarian function and induce permanent or transient amenorrhea in one-to two-thirds of women. Suppression of ovarian function is effective therapy for premenopausal women with hormone receptor-positive breast cancer. Aromatase inhibition is effective for postmenopausal women with hormone receptor-positive breast cancer, but is not indicated in premenopausal women. Objective: We aim to determine whether, and for how long, ovarian estrogen production persists after chemotherapy-induced amenorrhea. Materials & methods: Serum estradiol, follicle-stimulating hormone and luteinizing hormone levels were determined in 56 breast cancer patients, 1-34 months after the onset of chemotherapyinduced amenorrhea. In 18 patients, these values were determined more than once. Postmenopausal estradiol levels are less than 10 pg/ml, follicle-stimulating hormone and luteinizing hormone greater than 20 mIU/ml. Results: The estradiol level was greater than 9 pg/ml at least once in 42 out of 56 patients. For 75 determinations, the estradiol level ranged from less than 10 to 344 pg/ml, with a median of 25 pg/ml and a mean of 41 pg/ml. Estradiol levels were not significantly correlated with either the number of months after the onset of amenorrhea at which they were drawn, or follicle-stimulating and luteinizing hormone levels. Conclusions: Ovarian estradiol production often persists for 1 year or more after chemotherapy-induced amenorrhea.

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