Computer-aided systems have been developed recently to improve the precision of implantation of unicompartmental or total knee replacements. Minimally invasive techniques were developed to decrease the surgical trauma related to prosthesis implantation. However, there are concerns about loss of implant positioning accuracy with minimally invasive techniques. Minimally invasive instruments have been adapted for use with a typical 6-cm skin incision for unicompartmental knee replacement. We prospectively studied 60 patients who had minimally invasive navigated UKA and compared them with an earlier group of 60 patients who underwent open navigated UKA. We used an intraoperative non-image-based navigation system. Minimally invasive navigated implantation of a UKA did not reduce the radio-graphic accuracy of the implantation compared to open navigated implantation. There were no major complications and little change from the conventional navigated operating technique. Because we do not yet know if navigation influences function and long-term survival, our conclusions need to be confirmed on a larger scale.