The Madigan Endoscopic Sinus Surgery (ESS) Simulator, developed by a multi-institution team led by Lockheed Martin, includes force-feedback instrument and virtual endoscope interaction with three-dimensional paranasal anatomy models derived from the Visible Human dataset, supplemented by a variety of graphical and auditory instructional aids embedded in the model. Our formal evaluation of Version 1.2 of the system focused on its validity as an ESS simulator. Run-time and survey data were collected for three groups of subjects on a common protocol progressing through the three basic ESS subtasks: navigation, injection, and dissection. Non-MD subjects performed the tasks on a simplified abstract virtual model with instructional aids (hoops for the navigation path, injection targets, dissection spheres, auditory feedback about task completion, and simulated patient heart rhythm). Non-ENT MDs progressed from this "novice" model to a simulated anterior ethmoidectomy on an "intermediate" model with the aids embedded in the reconstructed and segmented paranasal anatomy. Otolaryngologists ranging from second-year ENT residents through senior staff progressed through both the abstract and intermediate models, and then performed the simulated surgical procedure on an "advanced" model, consisting of the anatomy with no instructional aids. The procedural validity of the simulator is supported by a strong correlation between performance on the simulator and degree of prior ESS experience, by convergent correlation among independent measures of subject task performance, and by subjective evaluations by experienced ESS surgeons.