We studied a 13 1/2-yr-old boy with severe juvenile osteoporosis and multiple metaphyseal and vertebral fractures. Biochemically, there was evidence of non-PTH mediated excessive bone resorption, low intestinal calcium absorption, and a strikingly negative calcium balance. He was treated with (3-amino-1-hydroxypropylidene)-1,1-bisphosphonate, a bisphosphonate capable of inhibiting bone resorption rapidly, and had dramatic clinical and biochemical improvement. All indices of resorption were normal within a week after initiation of therapy and his 1,25-dihydroxyvitamin D concentration, which was only 9.6 pg/ml before treatment, rose to 62.4 pg/ml. These changes were associated with an increase in calcium absorption and positive calcium balance. Radiological improvement with healing of metaphyseal and one diaphyseal fractures and signs of sclerosis near the growth plates of the affected metaphyses and at the end plates of the vertebrae also occurred. (3-Amino-1-hydroxypropylidene)-1,1-bisphosphonate, therefore, with its rapid suppression of resorption and the accompanying hormonal changes, is a very effective treatment for juvenile osteoporosis. The primary defect of this obscure syndrome seems to be uncontrolled activity of metaphyseal osteoclasts; disturbances of vitamin D metabolism and of intestinal calcium absorption are secondary events.