The results in 53 knees that had been treated by proximal tibial opening-wedge osteotomy for large varus deformity and osteoarthritis of the medial compartment were evaluated after a mean length of follow-up of ten years (range, 8–12 years). We used a porous beta-tricalcium phosphate (β-TCP) wedge because it is resorbable and osteoinductive. All osteotomies were completely consolidated and complete osseointegration of the remnant of the β-TCP wedge took place. However, after a mean maximum follow-up of ten years none of the cases showed complete resorption. After ten years, 40 (81%) of the 53 knees had an excellent or good result, and in 13 knees there was recurrent pain for which six had an arthroplasty. Although the results deteriorated with time, time was not the only determinant of the result. Alignment, measured as the hip-knee-ankle angle on radiographs of the whole limb that were made with the patient bearing weight, was also a determinant of long-term results. The best results were obtained in the knees that had a hip-knee-ankle angle of 183–186 degrees. In these knees, there was no pain and no progression of the arthrosis in either the medial or the lateral tibiofemoral compartment. Of the three knees that had an angle of more than 186 degrees, all five had progressive degenerative changes in the lateral compartment. In the undercorrected knees (an angle of less than 183 degrees), the results were less satisfactory, and there was a tendency toward recurrence of the varus deformity and progression of the arthritis of the medial compartment. However, when the correction was insufficient the deterioration was slow. Therefore, proximal tibial osteotomy is a very suitable operation even for patients who have gonarthrosis of the medial compartment and a large varus deformity. Although, a rigidly standardised and precise operative technique is required as well as accurate radiographic measurements of the mechanical axis of the limb because exact postoperative alignment is the prerequisite for the longest possible period of relief of symptoms after osteotomy, and this exact alignment is difficult to obtain for patients with large varus deformity.