Anorexia nervosa is associated with bone loss during adulthood, but may also delay skeletal growth and mineral accrual during growth. We asked the following questions. 1) Is anorexia nervosa associated with reduced bone size and reduced volumetric bone mineral density (vBMD)? 2) Is estrogen replacement therapy (ERT) or recovery from anorexia nervosa associated with normal bone size and vBMD? Using dual-energy x-ray absorptiometry, we measured bone size and vBMD of the third lumbar vertebra and femoral neck in a cross-sectional study of 161 female patients: 77 with untreated anorexia nervosa, 58 with anorexia nervosa receiving ERT, 26 recovered from anorexia nervosa, and 205 healthy age-matched controls. Results were expressed as the SD or z-score (mean +/- SEM). Deficits in vertebral body and femoral neck width in untreated women were -1.0 +/- 0.1 and -0.3 +/- 0.1 SD (P < 0.001 and P < 0.05, respectively). Deficits in bone width were less in the ERT-treated women than in untreated women at the vertebral body (-0.6 +/- 0.1 SD; P < 0.001), but not at the femoral neck (-0.4 +/- 0.2 SD; P < 0.05). There were no significant deficits in vertebral body and femoral neck width in recovered women (both -0.3 +/- 0.2 SD; P = NS). In untreated women, vertebral and femoral neck vBMD were -1.6 +/- 0.1 and -1.1 +/- 0.1 SD, respectively (both P < 0.001), less severely reduced in ERT-treated women (-1.2 +/- 0.2 and -0.6 +/- 0.2 SD, respectively; both P < 0.001), and least reduced in recovered women (-0.6 +/- 0.1 and -0.5 +/- 0.2 SD; P < 0.01 and P < 0.05, respectively). After adjusting for differences in fat and lean mass, vertebral body and femoral neck width were no longer reduced in untreated, ERT-treated, and recovered women. Adjustment for body composition had little effect on group difference in vBMD. Bone fragility in anorexia nervosa is due to reduced bone size and reduced vBMD. Although causality cannot be inferred in cross-sectional studies, the data are consistent with the view that malnutrition may contribute to reduced bone size, whereas estrogen deficiency may reduce vBMD. The use of ERT early in disease is a reasonable component of management if the chance of recovery appears remote.