Osteoporosis associated with anorexia nervosa (AN) is common, and tends to be severe, slow to recover from, and sometimes irreversible. The abnormal bone metabolism in severely emaciated AN patients involves both a reduction in bone formation and an increase in bone resorption. The annual change in lumbar bone mineral density (BMD) is significantly correlated with BMI at the entry. The critical BMI for a positive increase in BMD was 16.4±0.3 kg/m(2). Nutritional improvement with body weight gain is the most important goal of treatment for AN-related osteoporosis since it increases both serum levels of insulin-like growth factor-I, a potent osteogenic factor, and estradiol, a powerful bone resorption inhibitor. However, it is difficult for AN patients to accept weight gain. About 50% of AN patients are insufficient of vitamin D and 43% show an increase in plasma undercalboxylated osteocalcin, indicating a deficiency state of the vitamin K(2). Vitamin D(3) or vitamin K(2) (menatetrenone) can prevent further bone loss in severely emaciated AN patients. Recently, bone strength has been evaluated by both BMD and bone quality. Plasma levels of homocysteine, a marker of degradation of bone quality, have significantly positive correlation with their ages of AN patients. We must evaluate bone density as well as bone quality in AN patients.