Although opening-wedge high tibial osteotomy (OWHTO) has been reported the good clinical outcome, age is recognized one of the critical factors for affecting the outcome. In addition, the critical factors for affecting clinical outcome after OWHTO especially in elder patients were still under discussion. The purpose of this study was to evaluate the clinical outcome after opening-wedge high tibial osteotomy (OWHTO) and to determine the critical factors for a poor clinical outcome after OWHTO in patients aged over 65 years. Of the 233 patients who underwent OWHTO, 88 patients (36 males and 52 females) who were more than 65 years of age were enrolled in this study. The inclusion criteria were that follow-up was over 2 years after surgery and a correction loss after OWHTO was less than 3 degrees. Radiographic analysis was performed pre- and postoperative pelvic inclination (PI) according to the method of Schwarz et al. and weight bearing line ratio (WBLR). Knee function was measured pre- and postoperative extension and flexion angle. Clinical outcome was evaluated with pre- and postoperative Lysholm score. As a result, the preoperative WBLR was significantly changed and Lysholm score improved (p<0.0001), whereas the PI, knee extension and ROM were not changed after OWHTO. Regarding the essential factors for affecting clinical outcome after OWHTO, age and delta PI were negative, whereas preoperative WBLR, postoperative ROM, especially extension, had a positive effect (p<0.05). Furthermore, only delta PI had affected the improvement of clinical outcome with OWHTO (p<0.01), and postoperative knee extension was negatively correlated with the progression of pelvic retroversion (p< 0.01). In conclusion, age at surgery and progression of pelvic retroversion were the critical factors for poor postoperative clinical outcome after OWHTO. Care should be taken for the progression of pelvic retroversion after OWHTO because it deteriorates the clinical outcome by inducing the knee flexion contracture as the compensatory mechanism for the balance of sagittal alignment.