The results in ninety-three knees that had been treated by proximal tibial opening-wedge osteotomy for varus deformity and osteoarthritis of the medial compartment were evaluated after a mean length of follow-up of 11.5 years (range, ten to thirteen years). After ten years, only forty-two (45 per cent) of the ninety-three knees had an excellent or good result, and in fifty-one knees there was recurrent pain for which seventeen had another operation. At five years, on the other hand, 90 per cent of the knees had a good result. Deterioration occurred at an average of seven years after the osteotomy and was always associated with recurrence of pain. 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 twenty knees that had a hip-knee-ankle angle of 183 to 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 five knees that had an angle of more than 186 degrees, all five had progressive degenerative changes in the lateral compartment. In the sixty-eight 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 (average, seven years), and it was not associated with lateral laxity and deterioration of the lateral compartment, which are the changes that characterize the natural course of gonarthrosis as described by Hernborg and Nilsson. Therefore, proximal tibial osteotomy is a very suitable operation for patients who have gonarthrosis of the medial compartment, but a rigidly standardized 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.