Polysulfone (PS) dialysis membranes hydrophilized by blending poly vinylpyrrolidone (PVP) are well known to have excellent biocompatibility in clinical use. The objective of the present study is thus to clarify how PVP improves biocompatibility of PS membranes and furthermore to develop a patient-friendly PS dialysis membrane with higher biocompatibility. Biocompatibility based on both lactate dehydrogenase (LDH) activity and amount of protein adsorption was greatly different among four commercially available PS hollow-fiber dialysis membranes. PVP present on the inner surface of the hollow fiber was quantitatively determined by X-ray photoelectron spectroscopy (XPS), demonstrating the amount of PVP to be varying for each membrane. Structure parameters such as surface roughness, three-dimensional surface area and polymer particle diameter, indications of the physicochemical properties of the membranes, were measured on the observed inner surface images in both wet and dry conditions by atomic force microscopy (AFM) to account for dependence of biocompatibility on these structure parameters. The higher regularity polymer particle structure has in the wet condition, the lower wet/dry ratio surface roughness has and the larger wet/dry ratio polymer particle diameter has, that is, the more greatly the polymer particles swell by wetting, the higher biocompatibility is achieved by "cushion effect". (C) 2004 Elsevier B.V. All rights reserved.