A 41-year-old woman with a history of endometriosis had breast carcinoma di
agnosed and treated with tamoxifen. A pelvic mass was subsequently diagnose
d and observed on serial ultrasonographic examinations to increase in size.
At surgery extensive endometriosis was found. Exacerbation of endometriosi
s by tamoxifen may become more common as tamoxifen is used more frequently
to reduce the risk of breast cancer among premenopausal women.