A three-dimensional, single-phase, isothermal numerical model of polymer electrolyte fuel cell (PEFC) was employed to investigate effects of electron transport through the gas diffusion layer (GDL) for the first time. An electron transport equation was additionally solved in the catalyst and gas diffusion layers, as well as in the current collector. It was found that the lateral electronic resistance of GDL, which is affected by the electronic conductivity, GDL thickness, and gas channel width, played a critical role in determining the current distribution and cell performance. Under fully-humidified gas feed in the anode and cathode, both oxygen and lateral electron transport in GDL dictated the current distribution. The lateral electronic resistance dominated the current distribution at high cell voltages, while the oxygen concentration played a more decisive role at low cell voltages. With reduced GDL thickness, the effect of the lateral electronic resistance on the current distribution and cell performance became even stronger, because the cross-sectional area of GDL for lateral electron transport was smaller. Inclusion of GDL electron transport enabled the thickness of GDL and widths of the gas channel and current collecting land to be optimized for better current distribution and cell performance. In addition, the present model enables: (i) direct incorporation of contact resistances emerging from GDL/catalyzed membrane or GDL/land interface, (ii) implementation of the total current as a more useful boundary condition than the constant cell voltage, and (iii) stack modeling with cells connected in series and hence having the identical total current. (C) 2004 The Electrochemical Society.