Simulations are often used to model polymer flow during injection molding to design molds and select processing parameters. It is difficult to determine the accuracy of these simulations due to a lack of experimentally measured in-mold velocimetry and melt-front progression data. This article compares the results from commercial mold-filling simulation software to experimental data obtained via particle image velocimetry (PIV) in a special optical-access mold with a rectangular cavity. Moldflow was used to simulate the mold filling by a polystyrene melt in the experimental configuration, and these simulated results are compared to the appropriately averaged time-varying velocity field measurements. Simulated results for melt-front progression are also compared with experimentally observed flow fronts. The ratio of the experimentally measured average velocity magnitudes to the simulation magnitudes was found on average to be 0.99 with a standard deviation of 0.25, and the difference in velocity orientations was found to be 0.9 degrees with a standard deviation of 3.2 degrees. The corner area opposite the gate was most problematic for the simulation. The region behind the front also had a relatively high simulation error, though not as severe as that in the corner. POLYM. ENG. SCI. 2013. (c) 2012 Society of Plastics Engineers