This study prospectively evaluated the rate of biologic response to desmopressin (DDAVP) in 66 patients with type 1 or 2 von Willebrand disease (VWD), each of whom had, on the basis of available records, a clinically significant bleeding history and at least one of the following laboratory abnormalities: bleeding time (BT) longer than 15 minutes, ristocetin cofactor activity (VWF:RCo) less than 10 IU/dL, factor VIII coagulant activity (FVIII:C) less than 20 IU/dL (severe VWD). Before the study, responsive patients were defined as those who, 2 hours after infusion of 0.3 microg/kg DDAVP, had increased baseline values of VWF:RCo and FVIII:C by at least 3-fold and achieved levels of at least 30 IU/dL for both and a BT of 12 minutes or less. The rate of biologic response varied according to VWD types and was higher in type 1 (7 of 26, 27%) than in type 2 (7 of 40, 18%) (type 2A [1 of 15, 7%], type 2M [3 of 21, 14%], type 2N [3 of 4, 75%]). Mutations in the VWF gene were previously known or newly identified in most patients with types 2A (n = 15 of 15), 2M (n = 15 of 21), and 2N (n = 4 of 4), but in none of those with type 1 VWD. Genotype provided more information than phenotype in predicting individual responses to DDAVP only in patients with 2A and 2N VWD. This prospective study showed that the rate of biologic response to DDAVP is relatively low not only in type 2 but also in type 1 VWD when uniform and stringent criteria for patient selection and responsiveness are applied.