Flooding in Polymer Electrolyte Membrane (PEM) fuel cells was studied with a single channel cell oriented horizontally and vertically. Water formed at the cathode/membrane interface flowed through the largest pores in the GDL when driven by a hydraulic pressure sufficient to overcome the hydrophobic surface energy of the GDL's pores. When the cathode flow channel was horizontal with the cathode facing down the liquid water falls away from the GDL and collected on the wall of the flow channel and was pushed out with minimal effect on the local current density in the fuel cell. When the cathode faced up the water slugs accumulated on the GDL surface downstream in the flow channel causing the local current density to periodically fluctuate as water slugs accumulated and were pushed from the flow channel. When the gas flows were downward, the local fuel cell currents were stable. Flooding in PEM fuel cells was gravity dependent and the local current densities depended on dynamics of liquid slugs moving through the flow channels. This is an abstract of a paper presented at the AIChE Annual Meeting (Salt Lake City, UT 11/4-9/2007).