Summary: A randomized controlled trial was undertaken comparing the efficacy and safety of low molecular weight (LMW) heparin (Fragmin) with sodium heparin for prophylaxis against postoperative thromboembolic disease after major gynaecological surgery. Women were randomized to receive subcutaneous injections of 5,000 U of either once daily LMW heparin or twice daily sodium heparin. A total of 566 women were recruited, of whom 552 completed the study. Most women (461) had malignant disease and 430 of these underwent radical surgery. The remainder underwent major, but not radical surgery. There were 5 thromboembolic events in the LMW heparin group and 2 in the sodium heparin group, with no significant difference between these groups. No significant difference was found in the incidence of intraoperative or postoperative transfusion in the 2 groups. The decision of which heparin to use in routine practice cannot be made on clinical grounds.