Prevention of venous thromboembolism (VTE) can be achieved through mechanic or pharmacological means. For the latter, unfractionated low dose heparin, low molecular weight heparins and oral anticoagulants are successfully and widely employed. Results of controlled and uncontrolled studies favour the use of prophylactic heparin in different clinical and surgical conditions such as myocardial infarction, stroke, orthopedic or prolonged surgery and surgical interventions in patients older than forty. Useful parameters to evaluate the results of VTE prophylaxis are discussed as well as timing, duration, effectiveness, side effects and costs of therapy. Although the benefits of VTE prophylaxis in high risk patients are clear, it is not routinely employed in Argentina.