We present preliminary work on a novel technique for tracking anatomical structures during medical procedures. A vascular balloon catheter is placed within a vessel in the structure of interest and is inflated using a radioopaque contrast material. The balloon catheter is tracked over time using a fluoroscopy system, and three parameters of motion are determined (two of translation, one of rotation) for each view via analysis of the balloon image. These methods are applied to three patient data sets to estimate liver motion during a respiratory cycle.