In 19 consecutive patients with medial gonarthrosis, an arthroscopic examination with a biopsy of the load-bearing cartilage in the medial femoral condyle was undertaken at the same time as a proximal tibial osteotomy. A follow-up arthroscopic biopsy was performed on an average of 2 years after the osteotomy. In 9 knees there was an improvement in the cartilage quality, 8 knees were unchanged, whereas 2 knees had deteriorated. Radiographically, 6 knees had improved, 11 were unchanged and 2 had deteriorated. We found no correlation between cartilage improvement and the clinical and radiographic outcome. Our results confirm that an osteotomy has a beneficial effect on the load-bearing cartilage in the medial femoral condyle.