Research has suggested that children with sufficient vitamin D levels undergoing hematopoietic stem cell transplant (HSCT) have improved outcomes, including lower incidences of infection and graft-versus-host disease (GVHD), as well as overall improved survival. However, supplementation in children undergoing HSCT has shown to be a challenge using standard or aggressive supplementation strategies. The overall objective of our study is to determine the effectiveness of a single, high dose oral vitamin D at the start of transplant followed by maintenance supplementation in children undergoing HSCT.