Abstract. We present a system that measures the probe location with respect to the patient's body during an ultrasound exam. The system uses an inexpensive trinocular camera to track patches of points on a surface. The accuracy of the digital camera system is measured, using the Optotrak as a reference, and is less than ±2 mm for a 20 × 20 mm patch. Fiducials, which can be seen in both the ultrasound images and the camera images, are used for a consistency test producing values with a standard deviation under 3.2 mm.