The evolution of mechanical stress in porous silicon films during drying was investigated. It was found that the dominant factor is capillary forces which art responsible for high tensile stresses during drying, and which lead to damage and destruction of highly porous films. Before drying, the films are found to be under compression. This can be related to the increased lattice parameter observed in porous silicon. After drying, the compressive stress is reduced. The difference in stress was ascribed to van der Waals forces which lead to attraction between the hydrogen-covered pore surfaces.