Nowadays, most patients under oral anticoagulant therapy (OAT) require invasive procedures such as colonoscopy (CC) or gastroscopy (GC). The goals of the management of OAT are to minimise the risk of thromboembolism and bleeding. We have performed the first prospective, observational study to evaluate these parameters using fixed-dose high-risk thromboprophylactic therapy with sodic bemiparin (Hibor) as bridging therapy. From January 2004 to January 2005, patients under OAT were included. Periprocedure prophylaxis consisted of: Acenocumarol patients: Day -3: withdrawal acenocumarol. Days -2,-1,0: Hibor 3500 UI/d sc and days +1,+2,+3: Hibor 3500 U/I + acenocumarol. And day +5: acenocumarol only. Warfarin patients: Days -5,-4: withdrawal warfarin, -3,-2,-1, 0; Hibor 3500 UI/day sc, days +1,+2,+3,+4: Hibor 3500 UI/day sc and warfarin and day +5; warfarin only. Thromboembolic complications and bleeding were recorded in a 3 month follow-up. We included 100 consecutive patients in the intention-to-treat group. The remaining 98 patients were 50 women and 48 men. Mean age of women was 71.1 (range: 46-87) years and 70.7 (range: 39-86) years in men. Eighty-three took acenocumarol, and 15 warfarin. Thirty-two gastroscopies and 61 colonoscopies were performed and in five patients both were performed. No thromboembolic and bleeding complications related to bemiparin were observed in the 103 endoscopies. Two patients developed pruritus at the punction site. Fixed-dose high-risk thromboprophilactic therapy with bemiparin (Hibor) is safe and effective as a bridging therapy in patients under OAT who require GC or CC.