Comorbidities and complications including infection, organ system toxicity, graft-versus-host disease (GVHD) and disease recurrence are some of the biggest contributors to quality of life and mortality in children undergoing hematopoietic stem cell transplant (HSCT). Research has suggested that patients with sufficient vitamin D levels during transplant have improved outcomes, including lower incidences of infection and acute GVHD, as well as overall improved survival. Prior research has shown that chronically ill children are at risk for vitamin D deficiency, including those undergoing HSCT. Data has shown populations with as many as 70% of HSCT patients have insufficient levels of vitamin D at time of transplant. While several studies have attempted methods of vitamin D supplementation in this subset of patients, there has not been success with either standard or aggressive supplementation strategies.