Our primary hypotheses are based on previous data referred to above. Our primary hypothesis is that a single, high dose of vitamin D followed by maintenance supplementation in children undergoing HSCT is a safe and efficacious way to replete vitamin D stores during HSCT through Day +100 of transplant. Our secondary hypothesis is that adequate vitamin D levels in patients undergoing HSCT have positive effect on the overall outcomes, including graft versus host disease, immune recovery, rejection, relapse, and infection rates.