In this prospective study high tibial osteotomy for medial gonarthrosis was performed in 95 patients (105 knee joints). The patients underwent simultaneously diagnostic and operative arthroscopic surgery of the knee joint. A follow-up arthroscopic examination could be performed in 75 patients (85 knee joints) at the time of implant removal. In group 1 (20 knee joints), the osteotomy was performed after diagnostic arthroscopy without arthroscopic operation of the knee joint. The fixation of the osteotomy was accomplished by staples, postoperative plaster fixation and physiotherapy. In group 2 (20 knee joints), osteotomy was performed without additional operative arthroscopy after diagnostic arthroscopy, internal fixation by AOT-plate, no external fixation postoperatively and physiotherapy. In group 3 (22 knee joints), osteotomy was performed with additional operative arthroscopy (Pridie drilling), internal fixation by AOT-plate no external fixation postoperatively no external fixation, physiotherapy and continuous passive motion. In group 4 (23 knee joints), osteotomy was performed with additional operative arthroscopy (abrasio-arthroplasty), internal fixation by AOT-plate, no external fixation postoperatively, physiotherapy and continuous passive motion. All patients underwent arthroscopic examination of the knee with cartilage biopsies taken from three different regions of the femoral condyle during the same operative session as the osteotomy. At follow-up arthroscopy cartilage biopsies were taken from the same regions. There was no great difference in clinical outcome after 1 year between all groups. Arthroscopy as well as routine and electron microscopy showed better cartilage regeneration in groups 3 and 4. Groups 1 and 2 showed only regeneration isles, sometimes not well fixed to the underlying bone, while in groups 3 and 4 cartilage regeneration was thicker and more stable, sometimes covering all of the pre-existing erosions. Therefore, we recommend osteotomy of the tibia for osteoarthritis together with operative arthroscopy in the same operative session.