Several studies have demonstrated that prolongation of thromboprophylaxis after elective hip replacement significantly reduces the frequency of venographically demonstrated deep vein thrombosis. This paper reports an economic evaluation of prolonged prophylaxis with low-molecular-weight heparin (LMWH), based on outcome data from one of these trials. Analysis showed a net saving per patient of 3,400 Swedish kronor. Consequently, if the costs of administering the LMWH -- which includes the cost of teaching the patient to self-administer whilst in hospital and the cost of a follow-up visit by a district nurse to ensure compliance -- are below this amount, the intervention will prove to be cost saving.