Because of recent concerns about the long-term risks of estrogen replacement therapy in postmenopausal women, there is growing interest in a group of compounds known as selective estrogen receptor modulators (SERMs). The SERMs bind to estrogen receptors and have tissue-specific effects that allow them to function as estrogen agonists in some tissues and estrogen antagonists in other tissues. There are four SERMs currently marketed in the United States. These include the triphenylethylenes--clomiphene citrate (Clomid), tamoxifen, and toremifene--and the benzothiophene, raloxifene. Clomid is used primarily in the treatment of infertility. Tamoxifen is indicated for the treatment and prevention of breast cancer. It has an estrogen antagonist effect on breast tissue, but an estrogen-like effect on lipids, bone, and the endometrium. Toremifene has an antagonist/agonist profile similar to that of tamoxifen. Raloxifene is approved for the prevention of osteoporosis in postmenopausal women. It is thought to be an estrogen antagonist on the uterus and breast tissues and an estrogen agonist with respect to bone and serum lipids.