For the next generation polymer electrolyte fuel cells (PEFCs), membrane electrode assemblies (MEAs) should be able to operate at high temperatures and low relative humidity (RH). Under such conditions, cell performance drastically decreases owing to the high resistance of proton conductivity in the proton exchange membrane (PEM). It is thus challenging to design a PEM that exhibits high proton conductivity at high temperature and low RH. In this study we propose a pore-filling membrane, composed of a low equivalent weight (EW) perfluorosulfonic acid (PFSA) polymer and a mechanically strong thin ultra-high molecular weight polyethylene porous substrate. This pore-filling membrane suppresses the swelling of the low EW PFSA polymer and exhibits high proton conductivity. Inside the pores of the substrate, a high-density structure of sulfonic acid groups was achieved, which is advantageous for high proton conductivity at low humidity conditions. The MEA with the pore-filling membrane shows high fuel cell performance at high temperatures and low RH; the maximum power density was around 1000 mW cm(-2) at 100 degrees C and 30% RH. Although the thickness of this membrane was around one-fourth of that with Nafion 211 (7 mu m), they have comparable hydrogen crossover current density owing to the effective swelling suppression.