Histomorphometric analysis of iliac crest biopsies was performed after tetracycline double-labelling in 22 hyperthyroid patients before and after medical antithyroid treatment for an average period of 4 months. The initially increased cortical porosity was normalized during treatment whereas the amount of trabecular bone was unchanged. The osteoclastic resorption in cortical bone decreased but was still elevated. The osteocytic osteolysis remained slightly increased. In trabecular bone, however, the bone turn-over decreased to a subnormal level following treatment and the surfaces were inactive in bone resorption and bone formation. An increase was observed in the amount, extent and width of osteoid seams due to an increase in the lifespan of bone forming sites and a prolongation of the maturity period of osteoid. The observed increased deposition of cortical bone after antithyroid treatment may explain the positive calcium balance in this period.