Vitamin D supplements are increasingly being recommended to healthy adult fracture patients without an osteoporotic injury. Although this is a relatively new practice pattern, the basis for this adjunct therapy is grounded in the high hypovitaminosis D prevalence rates (up to 75%) among healthy adult fracture patients, and the strong biologic rationale for the role of vitamin D in fracture healing. Briefly, experimental animal studies have demonstrated that the concentration of vitamin D metabolites is higher at a fracture callus compared to the uninjured contralateral bone, vitamin D supplementation leads to decreased time to union and increased callus vascularity, and increases mechanical bone strength compared to controls. While evidence to confirm that vitamin D supplementation improves fracture healing in clinical studies does not exist, the pre-clinical data are compelling and worthy of further investigation.