Desmopressin (DDAVP) has recently been found to improve hemostasis in patients with congenital or acquired disorders of coagulation and to reduce operative blood loss in patients with normal hemostasis undergoing certain surgical procedures. Despite its potent antidiuretic effect, severe hyponatremia after the intravenous administration of DDAVP is felt to be rare. We report four cases of severe hyponatremia with serious clinical sequelae occurring in patients with underlying coagulopathies who were treated prophylactically with DDAVP to improve hemostasis prior to surgical procedures. Each patient received multiple (3-22) doses of DDAVP and was given intravenous hydration with hypotonic solutions before developing clinical signs and laboratory evidence of hyponatremia. We believe that the risk of significant hyponatremia after treatment with intravenous DDAVP may be higher than is generally appreciated and that patients undergoing surgical procedures, who often receive multiple doses of DDAVP and intravenous hydration, are at particular risk for this complication. Hypotonic intravenous solutions should be avoided and serum sodium levels should be monitored frequently in those patients receiving multiple doses of DDAVP.