This study examined the changes in knee alignment after an open wedge high tibial osteotomy before and after weight-bearing. From 2004 to 2006, 36 high tibial osteotomies were performed to treat unicompartmental arthritis with a varus deformity. Thirteen patients without instability and with an accurate radiographic evaluation were included. The changes in the deviation of the mechanical axis and femorotibial angle were evaluated retrospectively using whole extremity radiographs immediately after surgery (supine position) and 4 months after surgery (weight-bearing position). In the nonweight-bearing radiograph obtained immediately after surgery, the mean deviation of the mechanical axis was 22% laterally and the mean femorotibial angle was valgus 8.9 degrees . The weight-bearing radiograph at 4 months after surgery showed that the former shifted laterally 34% and the latter shifted valgus 10.6 degrees . The changes in the mechanical axis and femorotibial angle were significant (P < 0.001). During open wedge high tibial osteotomy, the surgeon should consider the increase in deviation of the mechanical axis and femorotibial angle after weight-bearing.