Recently there has been discussion on the value of prolonged thromboprophylaxis after elective hip surgery to prevent thromboembolic complications up to one month. Continuing low molecular weight heparin for one month can significantly reduce the frequency of venographically-proven deep vein thrombosis (DVT). How this will influence health economics has hitherto not been evaluated. Data on costs and effect from a randomized comparison between 10 and 30 days of once daily low molecular weight heparin (enoxaparin) were used in a cost-effectiveness study. Data on treatment costs were based on patients actually treated for DVT within the randomized study. Various sensitivity analyses were performed. In the study both phlebographically detected and clinically manifested venous thromboembolism were registered. By using costs from the health care system in Sweden it could be shown that the cost was 6,075 Swedish Krona (SEK) per phlebographic DVT avoided and 13,184 Swedish Krona (SEK) per clinical DVT avoided. If more than 50% of the patients managed to self-administer the low molecular weight heparin injections, the use of prolonged prophylaxis is cost-effective. Prolonged prophylaxis with the low molecular weight heparin enoxaparin after elective hip arthroplasty is cost-effective provided that at least 50% of the patients can administer the subcutaneous injections themselves.