Osteoporosis is an uncommon disease in children and early adolescents. Patients have a low bone mineral density, develop fractures with minimal or no trauma, and frequently have a negative family history. The disease results from either diminished bone formation or increased bone removal (resorption). No specific drug therapy has been recommended for juvenile osteoporosis. Alendronate (Fosamax) is effective in inhibiting bone resorption, increasing BMD and reducing fractures in adults with postmenopausal osteoporosis, but have not become established therapies in children. In the present study, we plan to evaluate the safety and efficacy of Alendronate in 20 patients with juvenile osteoporosis in a two-year period. This is a randomized, double-blind, placebo-controlled protocol. In the year-1, 10 patients will be assigned to receive Alendronate and 10 patients placebo. In the year-2, patients will be crossed over to the second arm of the study. Those who received Alendronate in the year-1, will receive placebo in the second year and vice verse. The patients will have 5 visits, the initial screening visit followed by 4 post therapy visits in a six-month interval. Measurements include DXA bone density scan of spine and hip, urinalysis and blood work.