Numerical simulations of a 50 cm(2) polymer electrolyte fuel cell (PEFC) with 36 channels are carried out to study the lateral transport of moisture and reactant between two neighboring channels with counter flow on the cathode side. Massive computations with 2.7 million computational elements are performed to capture the intricate electrochemical and transport phenomena in PEFC in three-dimensions. Two cases are examined and compared. One is under the common assumption that molecular diffusion dominates the species transport in the gas diffusion layer (GDL) and thus, the lateral convection effect is neglected. The other case is to include the flow and convection effects in the GDL using a realistic permeability of the porous GDL. Numerical results elucidate the mechanism and extent of internal humidification induced by lateral moisture diffusion. In addition, it is found that given the typical GDL, there exists a large pressure drop between two adjacent channels in counter flow, one flowing in from the inlet and the other flowing out to the outlet, causing severe reactant bypass between the two flow paths of reverse direction. The bypass results in reactant flow "short-circuit" and greatly diminishes the internal humidification benefit intended by the counter-flow design. (c) 2005 Elsevier B.V. All rights reserved.