The relationship between malignancy and thrombosis and hemostasis has long been recognized but is poorly understood. The interaction of tumor cells and the proteins and cells of coagulation is symbiotic and complex. Selected cancers have been known to respond to antithrombotic and anticoagulant therapies such as warfarin, heparin and most recently, hirudin. The identification of the functional 'tethered ligand' thrombin receptor on platelets, other cells and recently tumor cells has provided additional opportunities to examine and control the course of tumor proliferation. Thrombin, at concentrations which precede fibrin formation, is a potent inducer of tumor cell expression of various integrin receptors and tumor cell adhesion to the matrix and other activated cells. New strategies based on specifically controlling the action of thrombin may be beneficial in arresting the proliferation of these cells.