Tamoxifen treatment for breast cancer may induce ovarian cysts and supraphysiological levels of serum estrogen. We report successful management with luteinizing hormone-releasing hormone (LHRH) agonist of ovarian hyperstimulation induced by tamoxifen. A 49-year-old woman was operated on for invasive ductal carcinoma of the right breast. She received breast irradiation and adjuvant tamoxifen therapy. After 2 years, she had a cystic ovarian mass, and her serum concentration of estradiol was 1280 pg/mL. She was treated with an injection of 11.25 mg leuprolide acetate, a long-acting LHRH agonist, without abandoning tamoxifen therapy. The levels of estradiol decreased to <10 pg/mL and the cystic mass disappeared 2 months later. Three-month depot treatment with LHRH agonists can be useful for patients receiving tamoxifen for breast cancer who have ovarian cysts and supraphysiological levels of estrogen.