A prospective, randomised, controlled trial compared two different techniques of high tibial osteotomy with a lateral closing wedge or a medial opening wedge, stabilised by a Puddu plate. The clinical outcome and radiological results were examined at one year. The primary outcome measure was the achievement of an overcorrection of valgus of 4 degrees . Secondary outcome measures were the severity of pain (visual analogue scale), knee function (Hospital for Special Surgery score), and walking distance. Between January 2001 and April 2004, 92 patients were randomised to one or other of the techniques. At follow-up at one year the post-operative hip-knee-ankle angle was 3.4 degrees (+/- 3.6 degrees SD) valgus after a closing wedge and 1.3 degrees (+/- 4.7 degrees SD) of valgus after an opening wedge. The adjusted mean difference of 2.1 degrees was significant (p = 0.02). The deviation from 4 degrees of valgus alignment was 2.7 degrees (+/- 2.4 degrees SD) in the closing wedge and 4.0 degrees (+/- 3.6 degrees sd) in the opening-wedge groups. The adjusted mean difference of 1.67 degrees was also significant (p = 0.01). The severity of pain, knee score and walking ability improved in both groups, but the difference was not significant. Because of pain, the staples required removal in 11 (23%) patients in the closing-wedge group and a Puddu plate was removed in 27 (60%) patients in the opening-wedge group. This difference was significant (p < 0.001). We conclude that closing-wedge osteotomy achieves a more accurate correction with less morbidity, although both techniques had improved the function of the knee at one year after the procedure.