It releases von Willebrand factor (VWF) from the endothelium and FVIII, thereby improving haemostasis.9–11 Although treatment is often effective, FVIII:C response to desmopressin exhibits a high variability between patients with haemophilia A. Contributing factors reported in the literature are age, baseline FVIII:C and the F8-gene mutation.12–14 However, these factors do not explain the observed variability entirely. This evidence concerns the gene VWF and hemophilia A.