An open study was carried out in 92 patients to assess the effectiveness of a new low molecular weight heparin in preventing thromboembolic events after gynaecological surgery. The drug was administered subcutaneously 2 hours before the operation and afterwards once a day for 7 days in doses of either 7500 U. aXa/day or 15,000 U. aXa/day according to thrombosis risk and other variables. Only 3 patients developed deep vein thrombosis and none pulmonary embolism. There was no evidence of increased haemorrhage risk and few haematomas were noted at the wound or injection site. Laboratory data showed a marked increase in plasma inhibition of activated Factor X from Day 1 and this increase was maintained throughout the study period.