CONTEXT: Raloxifene is a promising breast cancer prevention agent in postmenopausal women at increased risk for breast cancer. The effects of raloxifene in premenopausal women are unknown. OBJECTIVE: We evaluated the effect of raloxifene in premenopausal women at increased risk for breast cancer on bone mineral density (BMD). DESIGN: This was a phase II clinical trial. SETTING: This study was conducted at an academic medical center. PARTICIPANTS: Thirty-seven premenopausal women at increased risk for breast cancer enrolled in the trial. Thirty subjects began treatment and 27 were evaluable. INTERVENTION: Raloxifene (60 mg daily) and elemental calcium (500 mg daily) were given for 2 yr. Subjects were followed up off medications for 1 yr. MAIN OUTCOME MEASURE: The primary end point was the intrasubject percent change in BMD at 1 yr measured by dual-energy x-ray absorptiometry. RESULTS: The mean baseline lumbar spine density was 1.027 g/cm(2). Lumbar spine density decreased 2.3% at 1 yr (P < 0.00001) and 3.5% at 2 yr (P < .00001). Percent change from yr 2 to 3 was +1.4%. The mean baseline total hip bone density was 0.905 g/cm(2). Total hip density decreased 0.3% at 1 yr and 1.0% at 2 yr (P = 0.033). Percent change from yr 2 to 3 was +1.7%. CONCLUSIONS: Raloxifene use is associated with a decrease in BMD in premenopausal women at increased risk for breast cancer. The clinical significance of this decrease is unknown and is attenuated with stopping raloxifene.