A prospective randomised controlled trial in 500 patients over the age of 50 who were undergoing major surgery showed that low-dose subcutaneous heparin was an effective prophylactic measure against fatal pulmonary embolism. None of the 252 patients who received perioperative heparin cover died of fatal pulmonary embolism while eight of the 236 who did not receive heparin prophylaxis died of fatal pulmonary embolism. These results were statiscally significant (P less than 0.01).