The purpose of this study was to investigate the efficacy of oral alendronate for children with osteogenesis imperfecta. Nine boys and seven girls of average age 9.5 years were given oral alendronate for an average of 4 years. Fracture frequency decreased, and in more than half of the patients the ambulatory/mobility status improved. Bone mineral density improved significantly, and restoration of collapsed vertebral bodies was observed. Urinary excretion of calcium and a bone resorption marker decreased significantly. Iliac crest physis biopsy showed an expanded primary spongiosa area with numerous multinucleated cells. This study reveals that oral alendronate caused biochemical, radiologic, and histologic changes along with clinical improvement. Oral alendronate treatment, which is convenient for the school-age group, was found to be a tolerable and efficacious treatment for children with moderate or severe osteogenesis imperfecta.