Discontinuation of thromboprophylaxis a few days after surgery may unmask delayed hypercoagulability and contribute to late formation of deep venous thrombosis (DVT). To investigate whether thromboprophylaxis should be prolonged beyond the hospital stay, a prospective, double-blind randomised study was conducted in 308 patients. All patients received initial thromboprophylaxis with dalteparin, dextran and graded elastic stockings. On day 7, patients were randomised to receive dalteparin (Fragmin) 5000 i.u. once daily, or placebo, for 4 weeks. All patients were subjected to bilateral venography, perfusion ventilation scintigraphy and chest X-ray on days 7 and 35. Patients with venographically verified proximal DVT on day 7 were withdrawn from the randomised study to receive anticoagulant treatment. The overall prevalence of DVT on day 7 was 15.9%. On day 35, the prevalence of DVT was 31.7% in placebo-treated patients compared with 19.3% in dalteparin-treated patients (p = 0.034). The incidence of DVT from day 7 to day 35 was 25.8% in the placebo-treated group versus 11.8% in the dalteparin-treated group (p = 0.017). The incidence of symptomatic pulmonary embolism (PE) from day 7 to day 35 was 2.8% in the placebo-treated group compared with zero in the dalteparin-treated group. This included one patient who died from PE. No patients experienced serious complications related to the injections of dalteparin or placebo. This study shows that prolonged thromboprophylaxis with dalteparin. 5000 IU, once daily for 35 days significantly reduces the frequency of DVT and should be recommended for 5 weeks after hip replacement surgery.