The effect of antithyroid treatment on the disturbed calcium-phosphorus metabolism in hyperthyroidism was studied in 16 patients. Elevated serum concentrations and urinary excretions of calcium and phosphorus were almost normalized 4 weeks after the start of medical treatment. Serum immunoreactive parathyroid hormone was decreased in the hyperthyroid state and became normal after medical treatment. Serum alkaline phosphatase levels were elevated throughout the study with an increase to a maximum peak after 8 weeks of antithyroid treatment. Urinary hydroxyproline excretion was initially markedly increased and fell rapidly during therapy. The observed changes suggest decreased bone resorption and increased bone formation with deposition of bone mineral after antithyroid treatment. Alterations in the serum albumin concentration during the investigation period influenced the total serum calcium concentration. Using albumin adjusted serum calcium values no hypocalcaemia was found during medical treatment or after a subsequent subtotal thyroidectomy.