In this study, the three-dimensional transient cold-start model is applied to a real-scale polymer electrolyte fuel cell (PEFC) geometry, and transient cold-start simulations are carried out from subzero to normal temperatures. In order to reduce the computational turnaround time involved for a large numerical mesh with millions of grid points, the cold-start code is parallelized for parallel computing. The simulation results clearly show the evolution of ice, water content, temperature, and current density contours at different cold-start stages, characterizing freezing, melting, hydration, and dehydration processes. In addition, the model predictions emphasize the beneficial influence of vapor-phase diffusion from the cathode catalyst layer (CL) to the gas-diffusion layer (GDL) during cold starts, which can contribute to reducing ice accumulation in the cathode CL. As the effect of vapor-phase diffusion is substantial, more ice is accumulated in the cathode GDL than in the cathode CL. The total amount of ice accumulated inside a cell is therefore not always proportional to the amount of ice in the cathode CL, depending on the vapor-phase diffusion strength. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.