Venous thromboembolism is a frequent and potentially severe ailment in medical patients; the clinical signs are unreliable and as early detection of the thrombosis process by non-invasive techniques is not available, prevention appears to be an alternative. Careful definition of the medical situations at risk from venous thromboembolism is necessary. Age, prior history of venous thromboembolism, and immobilization constitute high risk circumstances. Analysis of the published studies advocates prevention in three circumstances: myocardial infarction, stroke and intensive care. In other cases, further controlled studies, randomized versus placebo, are needed.