We present a simulation for endoscopic procedures. The anatomy of the patient is represented by 3D polyhedral models, computed from tomographic images. The simulation program can be used as a training system, or for prior simulation of complicated individual cases to reduce the error rate and to decide on the feasibility of biopsies. Additionally, we show how the system could be used as an online monitor during the intervention without the use of external tracking devices. The parameters used are the inserted length and the axis-orientation of the endoscope. An example of a bronchoscopy is shown.