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 filters 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 (CVVH) with AN69. RESULTS. There was a rapid decrease of factor D levels to 62(+/-6%) of the pre-CVVH value during the first 2 h, which continued to 51(+/-7.3%) after 12 h; at 24 h there was a slight rise to 62 +/- 12%. Sequential use of Polyacrylonitrile (AN69) filters lowered factor D levels below the normal plasma concentration in three patients, thus producing a state of factor D depletion. CONCLUSION. The significant reduction of factor D levels during CVVH with PAN filters suggests that frequent changes of PAN filters may reduce alternative pathway function by lowering factor D levels. CVVH (as opposed to intermittent dialysis) with PAN membranes may further improve the outcome of critically ill patients.