Fifteen patients with varus gonarthrosis underwent high tibial osteotomy and internal fixation with an L-shaped rigid plate. In 9 patients, an average wedge size of 7.1 degrees was resected leaving the medial cortex of the proximal tibia intact (group 1). In 6 patients, the medial cortex of the proximal tibia was unintentionally fractured during surgery when an average 10.7 degrees wedge was resected (group 2). Postoperatively, patients were monitored with serial radiostereometric analysis (RSA), conventional radiographs, and clinical evaluation for 1 year. In group 2, RSA revealed a 1.3-mm increase in lateral displacement of the distal tibial segment within 3 weeks following surgery. Twelve weeks after surgery, micromotion between tibial segments was below the precision of the RSA setup in 14 of 15 patients. These findings indicate that in cases with larger wedge sizes (>8 degrees), fracture of the medial cortex of the proximal tibia was frequent and resulted in significant lateral displacement of the distal tibia relative to the tibial plateau. In such cases, prophylactic additional medial fixation rather than lateral L-plate fixation alone is advised to minimize the propensity for lateral displacement of the distal tibia and to avoid subsequent loss of correction.