This paper presents a new on-line automatic X-ray fluo-roscopic C-arm calibration method for intraoperative use. C-arm calibration is an essential prerequisite for accurate X-ray fluoroscopy-based navigation and image-based registration. Our method utilizes a custom-designed calibration ring with a two-plane pattern of fiducials that attaches to the C-arm image intensifier, and an on-line calibration algorithm. The algorithm is robust, fully automatic, and works with images containing anatomy and surgical instruments which cause fiducial occlusions. It consists of three steps: fiducial localization, distortion correction, and camera calibration. Our experimental results show submillimetric accuracy for calibration and tip localization with occluded fiducials.