The effects of glucocorticoids on calcium and bone metabolism were investigated in 11 children (aged 6 months to 13 years) who were treated with dexamethasone, prednisolone and depot-ACTH because of different disorders. Alkaline phosphatase activity and osteocalcin in serum, representing indices of osteoblastic bone synthesis, and urinary hydroxyproline in relation to creatinine in morning fasting urine specimens, an index of osteoclastic bone degradation, decreased by 53-61% from baseline (P less than 0.01), with a highly significant relationship of all 3 indices to each other. Additional influences of glucocorticoids were hyperphosphaturia due to decreased renal phosphate reabsorption not mediated by secondary hyperparathyroidism, as well as marked hypercalciuria. As the consequence of the present study the following prophylactic or therapeutic recommendations are given during steroid-treatment: 1. Approvement of the negative balance of calcium and phosphate by correcting the hypercalciuria with hydrochlorothiazide, and the hypophosphatemia with oral phosphate and 2. in elder children with osteoporosis, stimulation of the decreased osteoblastic bone formation by sodium fluoride.