Abstract Previous attempts to model the uniaxial compression response of flexible polyurethane foam using a hyperelastic model for rubber-like materials assumed a zero Poisson’s ratio in the model development. The consequences of relaxing this assumption are explored in this investigation. First, measurements are made of the material’s Poisson’s ratio at high compression levels. The measured values range between 0.5 at 5% compression and −0.05 at 66% compression. The experimental results are combined with those of other investigators and incorporated into a nonlinear viscoelastic model for modeling uniaxial compression behavior. It is found that relaxing the zero Poisson’s ratio assumption decreases the accuracy of the model predictions, indicating that an alternative model structure may be required.