Survivors of childhood cancers face a variety of long-term problems. The investigators' recent study found that osteoporosis and osteopenia were common among these patients. The factors leading to, as well as the best treatment option for, this morbidity are unclear. Bisphosphonates are currently the standard therapy for osteoporosis in the elderly. However, the efficacy and safety of bisphosphonates for treating osteoporosis in long-term cancer survivors have not been tested. The investigators hypothesize that alendronate, an orally active bisphosphonate, is efficacious and safe in the treatment of osteoporosis in these patients.