Anorexia nervosa (AN), a psychiatric disease characterized by chronic starvation, is complicated by severe bone loss (1), for which there is no effective, available therapy. Although bone resorption is markedly increased in these patients, estrogen is an ineffective anti-resorptive therapy in the setting of undernutrition. We hypothesized bisphosphonate administration would result in a decrease in bone resorption and an increase in bone density in women with AN and bone loss, despite undernutrition. We therefore administered risedronate 5 mg daily for nine months to 10 women with AN, all of whom had osteopenia (mean AP spine T score: -2.7 +/- 2) and compared NTX and bone density with baseline values and with those from available control data prospectively followed for the same time period. Bone density increased significantly in patients who received risedronate compared with controls and compared with baseline, despite lack of significant weight gain, for an increase of AP spine bone density of 4.1 +/- 1.6% at six months and 4.9 +/- 1.0% at nine months. Bone resorption, as measured by NTX, decreased 23.8% at one month and 29.6% at three months, from the high-normal to mid-normal range of young women. Our data suggest that risedronate 5 mg daily administered to women with AN and osteopenia may increase in bone density at the AP spine despite low weight. This is the first study to demonstrate marked increases in bone density in women with AN. Because of the lack of data regarding the safety of such medications in women of reproductive age, bisphosphonates are not approved in the U.S. for premenopausal women other than those receiving glucocorticoids. Further studies are needed to establish the efficacy and safety of bisphosphonate therapy in this population.