Avascular necrosis of the femoral head and collapse is a devastating condition in the pediatric patient population leading to degenerative arthritis and significant disability. Bisphosphonate therapy has been shown to be effective in the treatment of adults with avascular necrosis of the femoral head. For children and adolescents however, there is currently no effective treatment that prevents collapse of the femoral head. Some patients with proven avascular necrosis of the femoral epiphysis are treated with bisphosphonates (intravenous pamidronate) at the The Hospital for Sick Children. The main aim of this study is to prospectively collect data on HSC patients receiving bisphosphonate treatment and thereby report the effectiveness of this treatment in the prevention of femoral head collapse.