The effect on the bleeding time of heparin, followed by an infusion with desmopressin (DDAVP) 0.3 Μg/kg or placebo in a randomized, double-blind set-up, was investigated in 20 patients treated for acute venous thromboembolism. The bleeding time was on the average prolonged by 90% (95% confidence interval 46% -134%) after at least 1 day of treatment with heparin. At that point DDAVP shortened the bleeding time by 23% with a confidence interval of −35% to −11% whereas the effect of placebo was −2% with a confidence interval of −23% to +20%. There was also a shortening of the APTT after DDAVP but not after placebo. It is concluded that DDAVP induces a partial reversal of the effect of heparin on the bleeding time, and that DDAVP should be tried in case of hemorrhagic problems.