Background: Heparin-free dialysis (HFD), i.e. only priming of the dialyser and no systemic heparinization, might reduce the risk of bleeding in dialysis patients receiving oral anticoagulation. The efficacy of HFD with two different membranes [polyacrylonitrile (AN69ST) vs. polysulphone (FX100)] was therefore investigated. Methods: In a crossover design, 10 patients were treated with either membrane for 1 week (total 30 sessions). Weekly Kt/V and urea reduction rate (URR) were measured and thrombus formation evaluated by measurement of D-dimer and inspection of the air traps and dialyser. Results: Change of clotting markers was similar with both membranes. Complete thrombosis occurred in 3/30 patients with the AN69ST and in 1/30 patients with the FX100 membrane. Kt/V did not differ between the groups, but the URR was slightly higher with the FX100. Conclusion: HFD prevents dialyser occlusion in the majority of patients treated with oral anticoagulants. Both polysulphone and polyacrylonitrile membranes seem to be equally effective.