We report a case of torsion of an ovarian follicular cyst that developed du
ring treatment with tamoxifen for breast cancer. A 40-year-old Japanese wom
an was admitted complaining of acute lower abdominal pain. Eight months ear
lier, she had undergone a partial mastectomy and local irradiation for duct
al carcinoma of her left. breast, estrogen receptor-positive stage I (T-1a
N-1b M-0). The administration of tamoxifen, 20 mg/day, and doxifluridine, 6
00 mg/day, were started immediately postoperatively, Pelvic examination aft
er admission revealed the left ovarian cyst and enlarged uterus, Transvagin
al ultrasonography and computed tomography revealed a multilocular cystic m
ass in the pelvic cavity, The pathological diagnosis of the tumor after tot
al hysterectomy and bilateral salpingo-oophorectomy was a typical follicula
r cyst with torsion and uterine leiomyoma, This ovarian cyst was believed t
o have developed during tamoxifen administration.