Confined compression tests performed by the authors on open-cell polyurethane foams reveal the presence of strain localization. After a brief description of the experiments, a theoretical model is proposed. In the model, the foam is represented as a chain of elastic springs with a two-phase strain energy density, and the strain localization is due to a progressive collapse of the springs. The collapse is a sort of continuum instability, which can be attributed to phase transition. An appropriate choice of the material constants leads to a close reproduction of the experimental force-elongation response curves.