Summary: The accuracy of implantation is an accepted prognostic factor for the long-term survival of unicompartmental knee prostheses (UKP). We developed a non-image-guided navigation system for UKP implantation, suitable for either a conventional or a mini-invasive approach. Twenty patients operated with the conventional navigation system (group A), 20 patients operated with the mini-invasive navigated technique (group B), and 40 patients operated with the conventional manual technique (group C) were matched using age, gender, body mass index, preoperative coronal mechanical femorotibial angle, and severity of the preoperative degenerative changes. All patients had a complete radiologic examination in the first 3 months after the index procedure, with anterior-posterior (AP) and lateral plain knee x-rays and AP and lateral long leg x-rays. There was a significant increase of the rate of prostheses implanted in the desired angular range for all criteria except the coronal mechanical femorotibial angle in groups A and B. An optimal implantation with all optimal items was obtained in 13 of 20 cases in groups A and B and 12 of 40 cases in group C. There were no relevant differences between groups A and B. Navigated implantation of a UKP with the used, non-image-based system allowed us to improve the accuracy of the radiologic implantation without any significant inconvenience and with little change in the conventional operative technique. The mini-invasive technique was possible with the same precision. This improvement might be related to a longer survival of such implanted prostheses with faster recovery.