The aim of this prospective cohort study was to determine the incidence of dalteparin bioaccumulation (measured using anti-Xa levels), and bleeding during thromboprophylaxis in elderly patients with renal failure who were admitted to hospital with an acute medical illness. Patients who met the criteria for being at high thromboembolic risk received dalteparin 5,000 IU subcutaneously once daily while the other patients (low risk) received 2,500 IU daily. Thromboprophylaxis was administered for at least 6 days. Anti-Xa activity was determined before the first dalteparin dose and again on day 6, 4 hours after the administration of the dalteparin dose. Bleeding was assessed daily. Compression ultrasonography was performed to identify any deep vein thromboses. There was no evidence of bioaccumulation on day 6 of therapy, irrespective of renal function. No episodes of major bleeding or venous thromboembolism occurred. Larger, randomized studies are warranted to confirm the safety of dalteparin in this patient population.