A three-dimensional computational study based on the finite volume method is carried out for proton exchange membrane (PEM) fuel cells with a Nation 117 membrane and an interdigitated flow field on the cathode. Emphasis is placed on obtaining a fundamental understanding of fully three-dimensional flow in the air cathode and how it impacts the transport and electrochemical reaction processes. For the first time, fully three-dimensional results of the flow structure, species profiles and current distribution are presented for PEM fuel cells with the interdigitated flow field. The model results show that forced convection induced by the interdigitated flow field in the backing layer substantially improves mass transport of oxygen to, and water removal from, the reaction zone thus leading to a higher cell current density as compared to that of the serpentine flow field. The computations also indicate a need to account for water condensation and ensuing gas-liquid two-phase flow and transport in the porous cathode at high current densities. The present computer model can be used as a design or diagnostic tool for fuel cell cathodes with complex structural flow fields.