Our purpose in this work was to validate a very detailed Monte Carlo model, developed in MCNPX, of a Millennium 120 multileaf collimator integrated into a Varian Clinac 21EX treatment head. The Monte Carlo results were compared with measurements for both the 6-MV and 18-MV photon modes. The following comparisons were performed: depth-dose curves, lateral profiles, multileaf collimator leakage, the tongue-and-grove test, and the round leaf-end test. The good agreement between the Monte Carlo simulations and measurements showed that our model is accurate. Consequently, the benchmarks provided by our study can be used in future Monte Carlo studies.