A 500 W stack operating at medium temperatures was designed, manufactured and tested. Nanocomposite Nafion membranes/electrodes (MEA1-NZr) assemblies, containing a commercial yttria stabilized zirconia (YSZ) as a filler, were developed to work over 80 degrees C. A 100 cm(2) cell active area with a parallel serpentine flow field as reactants distributor was used for stack realization. Preliminary electrochemical tests in a single cell and in two-cells short stack were performed at 120 degrees C, 3 bar(abs) and fully hydrated gases, reaching a rated power at 50 A of about 30W in single cell and 70W in short stack. Finally, a 20 cells stack was assembled, with composite MEA-NZr, and tested at 120 degrees C, 3 bar(abs) and partially humidified gases. In these conditions, a power of 433 W at 50 A was reached. Comparing these results with the short stack performance at the same current (50 A), a 24% of power loss, which corresponds to 7 W/cell, was recorded. This performance reduction could be explained considering the scale up effect passing from 2-cells to 20-cells stack. The obtained results show that the developed composite membrane-electrodes assemblies and the designed stack are suitable for working at higher temperature than traditional polymer electrolyte membrane fuel cells. [DOI:10.1115/1.4005811]