Desmopressin [1-deamino-8-D-arginine vasopressin (DDAVP)] has been successfully used in the treatment of type 1 von Willebrand disease (VWD) and mild haemophilia A (MHA). Data suggest that DDAVP can increase factor XI (FXI) plasma levels and may represent an effective treatment for mild FXI deficiency. We assessed the DDAVP response of FXI coagulant activity (FXI:C), FXI antigen (FXI:Ag), factor V coagulant activity (FV:C), and factor X coagulant activity (FX:C) in 33 individuals with VWD or MHA. DDAVP did not produce a clinically significant increase in FXI:C, FXI:Ag, FX:C or FV:C in any patient. The mean ± SD FXI:C pre-DDAVP (time 0) and at 1 h post-DDAVP was 90·7 (±22·9) U/dl and 92·1 (±20·9) U/dl, respectively. The mean (±SD) FXI:Ag at time 0 and 1 h was 92·2 (±20·1) U/dl and 89·9 (±21·3) U/dl, respectively. There was a small reduction at 1 h post-DDAVP in both FV:C, from 101·8 (±20·9) U/dl to 97·2 (±21·4) U/dl (P < 0·001), and FX:C from 103 (±19·5) U/dl to 98·8 (±18·7) U/dl (P < 0·001). No significant increase in FXI:C, FXI:Ag, FV:C or FX:C levels was seen at 4 h post-DDAVP. This study failed to demonstrate a clinically significant increase in the levels of FXI, FX or FV following administration of DDAVP.