The use of template systems has aided the preoperative selection of correct prosthetic size during routine arthroplasty. A similar system exists for unicondylar knee arthroplasty. Our goal is to assess the reliability of these templates for preoperatively predicting the correct prosthetic size in unicompartmental knee systems. Ten observers estimated the size of the unicondylar knee prosthesis required for 30 randomly selected patients with osteoarthritis. Estimation of the size was gauged using templates and instructions provided by the manufacturer. The observers worked independently and repeated their measurements 2 weeks later. Intraobserver and interobserver agreement was evaluated using the weighted kappa coefficient, and this revealed poor agreement regardless of the surgeon's experience. This shows that the present system lacks reliability and raises concerns about the place for preoperative radiological templating in unicompartmental knee arthroplasty.