A fuel cell stack needs to be stable and high-performing for optimum commercial viability. A program was undertaken to evaluate stability of a number of proton exchange membrane (PEM) fuel cell stacks and systems by operating them as independent power units at the rated maximum power outputs. Eight convection/forced-convection stacks and systems ranging in power outputs from 3W to 150W were evaluated for periods ranging from 170h to 700h. One 300W forced-flow stack was evaluated for an 8-h period. All stacks and systems were operated self-humidified. The flow of hydrogen was kept dead-ended with periodic release to maximize its utilization. In general, the stability was observed to be excellent except of the smallest convection stack, which showed some variations from point to point. The documented stability behaviors indicate that stack and system designs were appropriate, the level of self-humidification was adequate, and that the tested products are ready for commercialization.