This study examined whether medial opening wedge osteotomy inadvertently changes posterior tibial slope and whether this change affects range of motion and functional outcomes. Lateral radiographs of 82 knees with varus arthrosis were reviewed to measure posterior tibial slope before and after medial opening wedge high tibial osteotomy. Anterior or posterior cruciate ligament-deficient knees were excluded. Twenty-one osteotomies were performed using distraction osteogenesis/medial external fixator, 26 using acute distraction/Arthrex plate fixation, and 35 using acute distraction/EBI plate fixation. Preoperative and postoperative Lysholm scores and range of motion were recorded. Posterior slope increased from a mean of 12.5 degrees preoperatively to 16.5 degrees postoperatively (P<.01). Fixation types revealed no difference in posterior slope change. Large slope changes had less preoperative knee flexion than did small changes (123 versus 131; P=.012). No significant correlation existed between posterior slope change and postoperative Lysholm scores (r=0.047, P>.05). We found that medial opening wedge osteotomy may alter sagittal alignment by increasing posterior tibial slope.