We present a new visualization system for image-guided stereotactic navigation in tumor surgery. The combination of frameless stereotactic localization technology with real-time video processing permits the visualization of medical imaging data as a video overlay during the actual surgical procedure. Virtual computer-generated anatomical structures are displayed intraoperatively in a semi-immersive head-up display. This results in surgical navigation assistance without limiting the judgement of the physician based on the continuous observation of the operating field. The case presented documents the potential of augmented reality visualization concepts in tumor surgery of the head.