Proximal tibial valgus osteotomy was performed for unicompartmental osteoarthritis in forty-five patients (fifty-one knees). The average age of the patients was forty-one years (range, twenty-three to fifty years), and the average length of follow-up was ten years. At follow-up, 70 per cent (thirty-six knees) were rated as good or excellent and 30 per cent (fifteen knees) were rated as fair or poor. There was no clear correlation between the quality of the result and the radiographic evidence of the severity of the arthritis preoperatively, the age of the patient at osteotomy, or the length of follow-up. There was a correlation between an improved result and an increased angle of correction after osteotomy, but the values were not statistically significant. The most important factor influencing the quality of results was the over-all level of disease in the knee as reflected in the preoperative knee score. Deficiency of the anterior cruciate ligament at the time of the osteotomy did not prevent a good result. We believe that proximal tibial osteotomy for unicompartmental arthritis of the knee is a good and effective procedure for patients who are less than fifty years old and who have an active life-style, and that lasting results can be achieved if the procedure is done early in the course of the disease.