We investigated whether the dosage of heparin injected subcutaneously for the postoperative prophylaxis of thromboembolism influences efficacy. We considered prospective, controlled or comparative, randomized studies in which heparin was administered in a dosage of 5,000 U b.i.d. or t.i.d. subcutaneously. In major surgical procedures in general surgery, gynecology, urology and chest surgery, 2 x 5,000 and 3 x 5,000 U of heparin/day lower the frequency of postoperative deep venous thrombosis from about 30% to about one-fourth and one-third that amount, respectively. With regard to orthopedic/traumatic surgery, in a study of only 40 patients, 2 x 5,000 U/day reduced the incidence of thrombosis by one-half. The use of the higher dosage resulted in a decrease in DVT in 5 of 7 reports, but the other two authors measured no prophylactic effect at all. Proof that subcutaneous heparin prophylaxis is also able to reduce the number of fatal postoperative pulmonary emboli has been produced only in the case of the higher dosage. In our own group of patients there is no correlation between body weight and frequency of hemorrhagic complications. In our patients there is no relation between malignant tumor as the primary disease and the occurrence of hemorrhagic complications. There is no evidence that the lower dosage causes fewer hemorrhagic complications than the higher dosage.