Osteopenia is a serious medical complication of anorexia nervosa, with no known effective treatment. We conducted a double-blinded, randomized trial comparing alendronate (10 mg daily) with placebo in 32 adolescents with anorexia nervosa (mean age, 16.9 +/- 1.9 yr). All subjects received 1200 mg elemental calcium and 400 IU vitamin D daily and received the same multidisciplinary treatment for their eating disorder. Bone mineral densities (BMDs) of the lumbar spine and femoral neck were measured by dual energy x-ray absorptiometry at baseline and after 1 yr of treatment. Twenty-nine subjects completed the study. Femoral neck and lumbar spine BMDs increased 4.4 +/- 6.4% and 3.5 +/- 4.6% in the alendronate group compared with increases of 2.3 +/- 6.9% and 2.2 + 6.1% in the control group (P = 0.41, femoral neck; P = 0.53, lumbar spine). From baseline to follow-up, BMD increased significantly at the femoral neck (P = 0.02) and lumbar spine (P = 0.02) in those receiving alendronate, but did not increase in those assigned placebo (P = 0.22, femoral neck; P = 0.18, lumbar spine). At follow-up, body weight was the most important determinant of BMD. BMD was significantly higher in subjects who were weight-restored compared with those who remained at low weight (P = 0.002, femoral neck; P = 0.04, lumbar spine). After controlling for body weight, treatment group assignment still had an independent effect at the femoral neck. We conclude that in adolescents with anorexia nervosa, weight restoration is the most important determinant of BMD, but treatment with alendronate did increase the BMD of the lumbar spine and femoral neck within the group receiving alendronate, but not compared with placebo in the primary analysis. Until additional studies have demonstrated efficacy and long-term safety, the use of alendronate in this population should be confined to controlled clinical trials.