Acute lymphoblastic leukemia is the most common form of childhood cancer with current treatment survival rates approaching 80%. Improved outcomes show an increased number of survivors at risk for long-term treatment related side effects including osteonecrosis. Osteonecrosis, or bone death, is caused by blood supply loss to the bone causing pain and poor quality of life. The hips, shoulders, knees and ankles may be affected. Pain is the usual presenting symptom and may become severe requiring surgical decompression or replacement of the affected joint. Long-term effects including arthritis and progressive joint difficulties will not be known for decades. This study aims to determine the risk factors for developing osteonecrosis that will lead to information for earlier detection and prevention. The study will be the basis for future intervention and prevention trials.