In a recent meta-analysis, low-dose heparin was shown to be effective in reducing the risk of venous thrombosis in patients undergoing general surgical procedures, in orthopedic patients, and in patients undergoing urologic procedures, with a risk reduction in each category of approximately 65%. Low molecular weight (LMW) heparins recently have been evaluated for the prevention of venous thromboembolism in randomized clinical trials in patients undergoing general surgical procedures, in orthopedic patients, in spinal cord-injured patients, and in medical patients, including patients with myocardial infarction, heart failure, respiratory failure, and patients with ischemic stroke. Data from the studies of patients undergoing general surgical procedures demonstrate that LMW heparins are effective in preventing deep vein thrombosis (DVT) and, in doses that have an equivalent antithrombotic effect as standard unfractionated heparin, the risk of bleeding complications is much less? Thus, the relative safety and efficacy profile is more favorable with LMW heparin than with unfractionated heparin in these patients. Patients undergoing orthopedic procedures provide a much more rigorous test of the efficacy and safety of LMW heparin fractions in DVT prevention. There is now solid evidence from randomized clinical trials that LMW heparins are highly effective in the prevention of DVT in high-risk orthopedic patients?