Epidemiologic data have shown that the risk of venous thromboembolism (VTE) in medical patients is comparable to the risk in surgical patients. Moreover, autopsy studies demonstrated that 75% of all in-hospital deaths for pulmonary embolism occur in nonsurgical patients. Hospitalized medical patients represent a very heterogeneous population. Patients with medical disorders who are at significant risk include those with heart failure, acute respiratory failure, acute infectious diseases, acute rheumatic disorders, inflammatory bowel diseases, and cancer. Recent consensus conferences have produced detailed recommendations for thrombosis prophylaxis in both surgical and medical patients. Both unfractionated heparin and low-molecular-weight heparins (LMWHs) have been shown to be effective when compared with placebo and are recommended in high-risk medical patients. When LMWHs were compared with unfractionated heparin, they were shown to have equivalent efficacy and greater safety than the parent compound. As for surgical patients, routine prophylaxis is still underused in the general medical population.