Every surgical procedure taxes the hemostatic defenses of the patient. If his hemostatic mechanism is sound, he is unlikely to have a bleeding problem during or after an operation, unless, of course, a suture or clip slips off. Two classes of patients do present bleeding problems to the surgeon. One group has a pre-existing bleeding tendency, the other acquires it during or after the operation. The recognition of patients with severe hemostatic disabilities, such as hemophilia, presents no problem since the patient is aware of the disease. The mild bleeder is less likely to be detected by screening tests than by adroit questioning. The major hemostatic defect that may develop during an operation, or shortly thereafter, is disseminated intravascular coagulation. This syndrome, always secondary, may accompany shock, mismatched blood transfusion, septicemia, or extensive malignancy. Its prevention or early recongnition is much easier than treatment after circulating platelets and some coagulation factors have been consumed and fibrinolysis is destroying fibrin and fibrinogen.