To assess the changes in bone mineral density and osteoblastic activity in patients with hyperthyroidism and to investigate the relationship between those changes, we measured bone mineral density and serum osteocalcin in 109 patients with Graves' disease, comprising 75 untreated patients and 34 patients under treatment, and 200 normal controls. The degree of change in bone mineral density was quantified with Z transformation, in which we used means and SDs of bone mineral densities obtained in the process of age- and sex-matched 1:1 pairing developed by ourselves. Bone mineral density was low in female patients with hyperthyroidism in the spine, femur neck, trochanter, and Ward's triangle, but was not low in male patients. Serum osteocalcin was elevated in patients with untreated Graves' disease and correlated negatively with the Z values of bone mineral densities of the spine, femur neck, and trochanter. In conclusion, indices of osteoblastic activity were elevated in patients with hyperthyroidism, probably secondary to a thyroid hormone-induced increase in bone resorption which resulted in reduced bone mineral density. Quantification of the change in bone mineral density by using the parameters derived in the process of age- and sex-matched pairing seems to be an efficient method for statistical analyses.