ABSTRACT The degree of osteoporosis in hyperthyroidism before and during treatment with carbimazole was studied by photon absorption technique of the right forearm and calcaneus. In addition serum total calcium, serum ionized calcium, serum phosphorus and serum alkaline phosphatase were determined. A group of 96 patients suffering from untreated hyperthyroidism (85 women and 11 men) was studied (79 of these patients were also followed during treatment) and compared to a control group of 157 persons (107 women and 50 men). The women were divided into two groups: ≤ 45 years old and more than 45 years old. In all groups untreated hyperthyroid patients showed lower bone densities compared to the control group, but this was only significant in women. During treatment all groups showed a significant increase in density. After 3–6 months of treatment bone density in the calcaneus increased 12 % and in the forearm 1.5 %; after 6 months – 3 years 33 % and 31 %, respectively. At that time bone density was normalized. There was no correlation between bone density in hyperthyroid patients and duration and severity of the disease. The biochemical changes were characterised by increases in serum alkaline phosphatase (26 %), serum total calcium (16 %) and serum ionized calcium concentration (17 %) in cases of untreated hyperthyroidism. Serum phosphorus concentration did not change. A correlation was found between elevation of the alkaline phosphatase and decreased bone density.