The purpose of this work was to provide an initial validation of a Monte Carlo (MC) model of the passive scattering treatment nozzle at the University of Texas M. D. Anderson Cancer Center Proton Therapy Center. The MC model included a detailed definition of each beam-modifying element in the nozzle, and calculations accounted for interactions of the beam with the rotating modulator wheel used to create the spread out Bragg peak. In this work we show comparisons of calculated dose and fluence profiles with measured data from the nozzle for the 250 and 180 MeV beam energies used for patient treatments. Agreement to within 1.5 mm of measured data was observed for all MC calculations. The high level of agreement between the measurements and the MC model for the two beam energies studied provides validation for use of the model in a study of the dosimetric effects of the proton beam size and shape at the nozzle entrance.