Cancer patients with solid tumors or hematologic malignancies receive most of their cancer treatment as outpatients. Thromboembolism, when it occurs, complicates the clinical care of these patients because of the associated morbidity and the need for anticoagulant treatment. It would seem that prevention of venous thromboembolism in these patients would be worthwhile. However, to date there are relatively few trials of anticoagulant prophylaxis in ambulatory patients with cancer and it is not possible to recommend prophylaxis for most patients. Research is needed to better define a population at high enough risk to warrant pharmacologic prophylaxis and to develop new antithrombotic agents for this indication.