Because estrogen deficiency predisposes to osteoporosis, we assessed the skeletal mass of women with anorexia nervosa, using direct photon absorptiometry to measure radial bone density in 18 anorectic women and 28 normal controls. The patients with anorexia had significantly reduced mean bone density as compared with the controls (0.64 +/- 0.06 vs. 0.72 +/- 0.04 g per square centimeter, P less than 0.001). Vertebral compression fractures developed in two patients, and bone biopsy in one of them demonstrated osteoporosis. Bone density in the patients was not related to the estradiol level (r = 0.02). Levels of parathyroid hormone, 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D were normal despite low calcium intakes. The patients with anorexia who reported a high physical activity level had a greater bone density than the patients who were less active (P less than 0.001); this difference could not be accounted for by differences in age, relative weight, duration of illness, or serum estradiol levels. The bone density of physically active patients did not differ from that of active or sedentary controls. We conclude that women with anorexia nervosa have a reduced bone mass due to osteoporosis, but that a high level of physical activity may protect their skeletons.