Heparin-induced thrombocytopenia develops when patients given heparin as treatment for thrombosis have an immunologic reaction to the agent. Patients may have an underlying thrombotic disorder--for example, venous thrombosis or pulmonary embolism--that requires intervention. Alternative antithrombotic therapies that have been tried include defibrinogenating agents, LMWH and heparinoids, thrombin-specific inhibitors, antiplatelet agents, oral anticoagulants, and vena cava interruption.