Single high-dose oral vitamin D therapy, known as stoss therapy, has been used in other chronically ill children where adequate levels of vitamin D are difficult to attain. Stoss therapy suggests a single high-dose followed by maintenance dosing would be adequate to replete and maintain vitamin D levels in chronically ill children. While it has been shown to be effective with no evidence of toxicity in patients with rickets and cystic fibrosis, its safety and efficacy has not been studied in the transplant setting. However, there is an urgent need to identify a modifiable factor may reduce the occurrence and/or severity of HSCT associated complications. Our overall objective of this study is to determine the effectiveness of a single, high dose oral vitamin D followed by maintenance supplementation in children undergoing HSCT. This change will result in a new and innovative approach to maintaining adequate vitamin D levels during pediatric HSCT, with the long term goal of reducing morbidity and mortality.