Desmopressin (DDAVP), a synthetic analogue of vasopressin has been successfully used in the treatment of type I von Willebrand's disease (VWD), mild factor VIII (FVIII) deficiency and intrinsic platelet function defects (PFDs) for almost three decades. However, there is limited published data documenting its efficacy and the reliability of circulating plasma FVIII:C as a surrogate marker of response to therapy in VWD. We report the haemostatic response to DDAVP in 133 consecutive patients (91 type I VWD, 20 mild FVIII deficiency and 22 PFDs). Minimal therapeutic response to DDAVP (0.3 microg/kg) was defined by normalization 30 min post- infusion of bleeding time for PFDs, factor VIII:C (FVIII:C) for mild haemophilia A, and von Willebrand factor antigen (VWF:Ag), von Willebrand factor functional activity (VWF:Ac) and FVIII:C for VWD. Nine out of 91 (10%) VWD patients failed to achieve minimal therapeutic response to DDAVP; plasma FVIII:C levels were an unreliable surrogate marker of DDAVP response as 6 out of 9 (67%) of these patients had normal post-infusion FVIII:C levels. Five out of the 20 (25%) patients with mild FVIII deficiency and 5 out of 22 (23%) patients with PFDs failed to achieve a minimal therapeutic response to DDAVP. DDAVP is an effective therapy in the majority of patients with type I VWD, PFDs and mild FVIII deficiency. The significant failure rate associated with this therapy supports the recent recommendations that response should be assessed in all patients at the time of diagnosis. FVIII:C is an unreliable guide of response to DDAVP in patients with VWD and therefore VWF:Ag and VWF:Ac should also be assessed. Failure to demonstrate the response of VWF:Ag, VWF:Ac and FVIII:C to DDAVP in patients with VWD is likely to increase the risk of haemorrhagic complications in patients with bleeding episodes or who are undergoing surgery.