We present the in-situ visualization of medical data taken from CT or MRI scans in real-time using a video see-through head mounted display (HMD). One of the challenges to improve acceptance of augmented reality (AR) for medical purpose is to overcome the misleading depth perception. This problem is caused by a restriction of such systems. Virtual entities of the AR scene can only be presented superimposed onto real imagery. Occlusion is the most effective depth cue [1] and let e.g. a correctly positioned visualization of the spinal column appear in front of the real skin. We present a technique to handle this problem and introduce a Virtual Window superimposed onto the real skin of the patient to create the feeling of getting a view on the inside of the patient. Due to motion of the observer the frame of the window covers and uncovers fragments of the visualized bones and tissue and enables the depth cues motion parallax and occlusion, which correct the perceptive misinformation. An earlier experiment has shown the perceptive advantage of the window. Therefore seven different visualization modes of the spinal column were evaluated regarding depth perception. This paper introduces the technical realization of the window.