A two phase nonisothermal 3D unsteady model is used to study the transients at start-up of a polymer electrolyte membrane fuel cell. The model is used to simulate start-up under different starting or initial conditions. The objective is to study the transient behavior of current and the phenomena affecting it. The transient current density obtained from simulation under purged and inflow/equilibrium initial conditions are plotted. The saturation and the temperature profile evolution within the gas diffusion layer under different conditions are also studied. The effect of gas diffusion layer thickness and reaction rate on the current density evolution is analyzed. It is found that the transient current density depends on the initial condition. Mass transport is the major phenomenon influencing the current density profile, and the mass transport transients are found to be subsecond in nature. The consumption and transport time scales are seen to affect the current undershoot at high loads. The liquid water evolution and distribution behaves very differently, under different initial conditions, as well as different inflow conditions. However, the total time taken by liquid water and temperature to reach steady state for different initial conditions is very close. It is also seen that the temperature transient is less than the liquid water transient, overall.