In a randomised clinical trial in 50 patients with symptomatic osteoarthritis of the medial compartment of the knee, the clinical results of high tibial osteotomy (HTO) according to the open wedge osteotomy (OWO) and closed wedge osteotomy (CWO) were compared. In both groups locked plate fixation was used. Clinical and radiological assessments were performed preoperatively and after one year. Postoperative hip-knee-ankle (HKA) correction angles were monitored on standing leg X-rays. The effect of HTO on collateral laxity of the knee was measured with a specially designed varus-valgus device. The WOMAC osteoarthritis index, the modified knee society score (KS) and visual analogue scales (VAS) were used to assess symptoms of osteoarthritis, function, pain and patient satisfaction. At one-year follow-up we found accurate corrections in both groups and the planned correction angles were achieved. No loss of correction was observed. Furthermore, the medial collateral laxity and the patellar height significantly decreased after OWO. Significant improvements of WOMAC and KS scores were found in both groups. All patients had significantly less pain and were very satisfied with the results. Surgery time was significantly longer in the CWO group, and complications were more frequent in this group. Both techniques led to good and comparable clinical results. The choice of whether to perform an open or a closed wedge osteotomy may be based on preoperative patellar height or concomitant collateral laxity.