Because protein adsorption depends significantly on the hydrophilicity-hydrophobicity of surfaces, it is important to evaluate nanoscale surface properties for improvement of the biocompatibility of dialysis membranes. The objective of the present study is to establish a nanoscale analytical method, which is independent of the test environment, for determining the hydrophilicity-hydrophobicity distribution on the inner surfaces of dialysis membranes. The hydrophobic adhesive forces to the wet dialysis membrane inner surfaces were determined using a hydrophobized atomic force microscopy probe, and the hydrophilicity-hydrophobicity distribution on the surfaces was visualized two-dimensionally at the nanoscale level. In addition, the relative intensity of hydrophobicity was examined by determining the ratio of the adhesive force to the test membrane inner surfaces to that to hydrophobized mica. The polysulfone (PSf) and polyester polymer alloy (PEPA) dialysis membranes decreased relative intensity of hydrophobicity on the inner surface with the addition of polyvinylpyrrolidone (PVP). The PEPA membrane inner surface had a more uniform hydrophilicity-hydrophobicity distribution with the PVP addition. On the other hand, the distribution on the PSf membrane inner surface remained uneven despite PVP addition. Our results demonstrate that it is possible to evaluate the hydrophilicity-hydrophobicity distributions on the surfaces of wet dialysis membranes using a hydrophobized probe. (C) 2011 Elsevier B. V. All rights reserved.