We have previously evaluated the safety and efficacy of Alendronate in 10 patients with juvenile osteoporosis during a 12-month clinical trial. We have documented that Alendronate improved BMD of the spine and hip without any major side effects. There were no additional fractures during therapy. The present study is designed to further evaluate the safety and efficacy of Alendronate in 20 children with juvenile osteoporosis using a double-blind, randomized, placebo-controlled, cross-over protocol.