There has been recent interest in using video cameras along with computer vision and photogrammetric techniques to aid in the daily positioning of patients for radiation therapy. We describe here a method to calibrate video cameras to the beam coordinate system of a radiation therapy treatment machine. Standard camera calibration relates the three-dimensional coordinate system of a calibration phantom to the two-dimensional image coordinates. Using a calibration phantom designed for simultaneous X-ray and video imaging, both types of images can be calibrated to a single coordinate system. A series of X-ray images of the calibration phantom is taken using the motion of the treatment machine. Camera calibration parameters derived from these images are used to find a transformation from the coordinate system of the calibration phantom to the beam coordinate system of the treatment machine. This transformation is applied to the video camera calibration to provide a camera calibration directly to the beam coordinates of the machine.