Pathology studies of human cancers suggested to early investigators that the hemostatic system may play an important role in cancer metastasis. Subsequent studies in animal models have demonstrated a reduction of tumor metastasis and improved animal survival with systemic anticoagulation. In many of these experiments, vitamin K antagonists (VKAs) were utilized. Although warfarin was effective in reducing metastasis in a majority of these animal models, effects on the growth of the primary tumor and on animal survival have been less consistent. Clinical studies on the effect of warfarin in human malignancy are limited and less than conclusive. Several small, uncontrolled and controlled clinical studies have been reported but do not definitively suggest a benefit in most malignancies. However, none of the studies of VKAs in humans are adequately designed or sufficiently powered to definitively exclude an impact of oral anticoagulants on cancer survival. Because of the difficulties in managing VKA oral anticoagulation in cancer patients and recent studies suggesting a positive effect on cancer survival with low-molecular-weight heparins, it unlikely that further studies on the use of VKAs in cancer patients will be undertaken.