Virtual colonoscopy is a non-invasive computerized medical procedure for examining the entire colon to detect polyps. We present an interactive virtual colonoscopy method, which uses a physicallybased camera control model and a hardware-assisted visibility algorithm. By employing a potential field and rigid body dynamics, our camera control supplies a convenient and intuitive mechanism for examining the colonic surface while avoiding collisions. Our Zbuffer-assisted visibility algorithm culls invisible regions based on their visibility through a chain of portals, thus providing interactive rendering speed. We demonstrate our method with experimental results on a plastic pipe phantom, the Visible Human, and several patients. CR Categories: I.3.3 [Picture/Image Generation]: Display Algorithms; I.3.5 [Computational Geometry and Object Modeling]: Physically Based Modeling; I.3.6 [Methodologies and Techniques]: Interaction Techniques; I.3.7 [Three-Dimensional Graphics and Realism]: Hidden Line/Surface Removal; I.3.8 [Applications];