Although deep vein thrombosis is often considered to be associated with recent surgery, 50-70% of symptomatic thrombembolic events occur in nonsurgical patients. Hospitalization for acute medical illness is independently associated with about eightfold increase in relative risk for venous thrombosis. Thus correct evaluation of risks for thrombembolic disease in individual inpatient and appropriate prophylaxis offers the opportunity to improve the prognosis of acutely ill patient. The most efficient and easy way of pharmacologic prophylaxis is subcutaneous administration of low molecular weight heparin. Only in minority of patients mechanical methods of prevention are sufficient. The optimal duration of thromboprohylaxis in medical patients is unknown and we have to evaluate the changing conditions in each patient.