Background. Polyacrylonitrile (PAN, AN69®) dialysis membranes have been shown to improve the outcome of critically ill patients. Factor D is an essential enzyme of the alternative pathway of complement and is increased during renal failure. On the other hand the contact of blood with biomaterials activates the complement cascade through the alternative pathway. PAN niters adsorb factor D which looses its enzymatic activity whilst bound to the membrane [1]; the complement alternative pathway function of serum exposed to PAN filters is greatly diminished and restored after addition of purified factor D [1]. The aim of our study was to measure the time course of factor D adsorption and its blood concentration during CVVH in critically ill patients with acute renal failure. Methods. We studied seven critically ill patients with ARF before, during and after continuous veno-venous haemofiltration (CWH) with AN69.