In a double-blind prospective clinical trial 75 consecutive patients undergoing mastectomy were randomly allocated to one of three groups. Twenty-five patients received perioperative anti-thromboembolic therapy with sodium heparin and 25 with calcium heparin. The remaining 25 patients were given anti-embolism stockings but no heparin. The total postoperative blood loss and period of drainage did not differ between the two groups given heparin, but a total of 10 of these 50 patients suffered haemorrhagic complications in the form of severe bruising or haematoma postoperatively. Of the patients given no heparin, none suffered haemorrhagic complications and the blood loss following the first 24-hour postoperative period was significantly less than in the groups given heparin.