The effect of extensional viscosity upon wall pressure and front progression in injection molding is presented through experiments and simulations. In order to compare materials having distinctly different responses to extensional flow, unfilled and glass-filled polypropylenes were examined. Wall pressure agreement was best with an extensional viscosity included in the simulation, particularly for the fined material. Pressure gradients were large enough to indicate significant density variations in the part at the end of filling. Examination of the test materials' viscoelastic properties indicates that extensional, rather than viscoelastic, effects predominate in the behaviors shown. Flow front progression was recorded by strobe photography. Results indicate that, when cavity thickness gradient is not aligned with the the direction of flow, there is disagreement among predictions and measurements. This is principally due to the violation of the Hele-Shaw assumption that forms the basis for the simulation.