BACKGROUND The effect of vitamin D on maintaining bone health is well researched and its role in the various metabolic stages of fracture healing has become evident, the clinical effects of a vitamin D deficiency in human fracture healing are less well described. Considering today's high prevalence of vitamin D deficiency, the aim of this present study was to investigate the effect of vitamin D status on human adult fracture healing. METHODS The serum calcidiol concentration was measured in a cohort of adult patients with a fracture in the upper or lower extremity between September 2012 and October 2013. Deficient patients (serum calcidiol <50 nmol/L) were treated with vitamin D. Clinical and radiological consolidation was determined. RESULTS Vitamin D concentration was measured in 617 patients; 249 (40%) were vitamin D deficient, they subsequently received vitamin D supplements. In 141 patients with a deficiency vitamin D was measured again after 4 months; 111 patients (78.7%) were no longer vitamin D deficient, and 30 remained vitamin D deficient. In six patients of the entire cohort, a clinical delayed union was described. The incidence of delayed union was higher in the group that remained vitamin D deficient (9.7%) compared to the patients who were not initially deficient (0.3%) and those who were no longer deficient after supplementation (1.7%; p < 0.001). The incidence of radiological delayed union did not differ between these three groups (p = 0.67). CONCLUSIONS The results of this research suggest that the vitamin D status at time of fracture affects fracture healing. Further research is needed to confirm these results.