We studied bone mineralization and calcium homeostasis in two children with hyperthyroidism before and during 3 yr of methimazole therapy in order to evaluate the effects of thyrotoxicosis and its therapy on mineral metabolism. Case 1, female, 4.1 year old with hyperthyroidism from 6 months. Biochemical data: increased thyroid function, phosphate and osteocalcin, decreased 1,25(OH)2D levels. X-ray: severe osteoporosis; bone mineral content (BMC) -23.0%, BMC/BW -25.1%. Case 2, female, 7.4 year old with hyperthyroidism from 9 months. Biochemical data: thyroid function, ionized calcium and osteocalcin were increased, 1,25(OH)2D and intact PTH were decreased. X-ray: severe osteoporosis: BMC -32.8%, BMC/BW -36.0. After the patients were euthyroid, they showed an increase of 1,25(OH)2D and intact PTH into normal values and a fall in calcium and phosphate. Osteocalcin levels returned in normal range one yr after first evaluation. Bone mineral analysis showed no variation of BMC and BMC/BW in the first 6 months of therapy and an increase in the following 6 months. In the following two years BMC and BMC/BW rose to normal range. Our study provides further evidence that in hyperthyroidism an altered mineral homeostasis is present with a reversible disturbance in vitamin D metabolism. We found that the return to euthyroidism was associated with a normalization of mineral homeostasis and with a recovery of bone mineralization. Osteocalcin assay may be an useful index to monitor bone metabolism in hyperthyroidism.