From 1960 through 1979, a closing-wedge varus osteotomy of the proximal part of the tibia was performed in thirty-one knees (twenty-eight patients) for painful osteoarthritis of the lateral compartment of the knee that was associated with a valgus deformity. The patients were followed for two to seventeen years (average, 9.4 years). Twenty-four knees (77 per cent) had either no pain or only occasional mild pain at the last evaluation. Six knees had moderate pain and one, severe pain. Six knees required a subsequent total knee arthroplasty at an average of 9.8 years after the osteotomy. No patient had an infection or non-union. Osteotomy of the proximal part of the tibia is a reasonable method of treating unicompartmental degenerative arthritis in a knee with a valgus deformity. Although some patients with as much as 20 degrees of anatomical valgus deformity obtained a good result in this series, osteotomy in the supracondylar region of the femur is probably preferable if the valgus angulation exceeds 12 degrees or if the tilt of the tibial articular surface that will result from the surgery will exceed 10 degrees. Correction beyond the normal 5 to 7 degrees of valgus angulation to zero degree of anatomical tibiofemoral alignment is recommended to prevent recurrence of the valgus deformity and to decrease the load on the lateral tibiofemoral compartment.