• Idiopathic juvenile osteoporosis is a rare cause of osteoporosis during childhood. We examined four children (three boys and one girl, ranging in age from 2.3 to 12.6 years) with idiopathic juvenile osteoporosis. All of these patients had normal serum calcium, ionized calcium, phosphate, magnesium, 25-hydroxyvitamin D, intact parathyroid hormone, and total and extractable calcitonin levels. 1,25-Dihydroxyvitamin D values were low in three patients and slightly decreased in one. Three children were treated with calcitriol (1,25 - dihydroxycholecalciferol) (0.50 μg/d in two and 0.25 μg/d in the other). The fourth patient was not treated because of parental refusal. Therapy reduced the fracture rate. Follow-up at 6 and 12 months showed a significant increase in bone mineralization, which reached normal values in two children after 12 months of treatment. No side effects of calcitriol therapy were observed. The untreated patient did not show an improvement of bone mineralization in the same time.