We present a novel approach for analyzing the experimental voltage-current curves of a polymer electrolyte membrane (PEM) fuel cell. State-of-the-art numerical models involve many poorly known parameters. This makes a comparison of numerical and experimental polarization curves unreliable. We suggest characterizing the cell by first using a simplified analytical model, which contains a minimal number of parameters and ignores three-dimensional (3D) effects. The resulting physical parameters are then used as input data for a 3D numerical simulation of the PEM fuel cell. Comparison of experimental, analytical, and numerical polarization curves enables us to estimate the contribution of 3D effects to the voltage loss. This procedure is performed using specially designed experiments, our recent analytical model, and the newest version of a numerical quasi-3D model of a cell. The results show that this approach may serve as a tool for the optimization of the flow field design. © 2005 The Electrochemical Society. All rights reserved.