Effect of desmopressin acetate on hemorrhage without identifiable cause in coronary bypass patients. After early hopeful reports, the ability of desmopressin acetate (DDAVP) to substantially reduce post surgical hemorrhage has been questioned. A total of 74 elective coronary bypass patients (Group A) receiving DDAVP (0.3 micrograms/kg) who, in the opinion of the operating surgeon, did not achieve adequate hemostasis after protamine neutralization of heparin were studied. They were compared with 91 age- and sex-matched controls (Group B). Before surgery there was no difference in hematocrit (40.8% vs. 40.3%); bleeding time (5.3 vs. 4.9 sec); platelet count (267 +/- 8 vs. 309 +/- 13 X 10(3)/mm3); fibrinogen (363 vs. 361 mg/dl); or activated clotting time (ACT) (168 +/- 4 vs. 163 +/- 3 sec). Both groups had the same number of grafts (3.3/pt), use of the mammary artery (72%), and average bypass time (124 min). There were also no differences in postbypass ACT (142 +/- 3 vs. 135 +/- 2 sec); platelet count (97 +/- 10 vs. 120 +/- 24 X 10(3)/mm3); and fibrinogen (157 +/- 35 vs. 207 +/- 40 mg/dl). However, postoperative hemorrhage was strikingly different: 1306 +/- 89 vs. 896 +/- 33 ml (P less than .0001). Fifteen patients in Group A bled more than 1.5 liters compared with 4 in Group B. Red cell transfusion rates were 1.23 +/- 0.26 for Group A and 0.35 +/- 0.8 for Group B (P less than 0.005). Sixteen Group A patients received additional blood products (plasma and platelets). The hemorrhage difference remained significant even when these patients were excluded (1098 +/- 57 vs. 896 +/- 34, P less than 0.003). Three Group A patients were re-explored without a bleeding source located. 