Fifty-one osteoarthritic knees treated by high tibial osteotomy were followed for at least five years. Deterioration after an initial good result was uncommon (five cases). Most of the good results were in knees (thirty of forty-five) with mild varus deformity and good ligament stability. It is, therefore, possible to select the knee suitable for osteotomy from measurement of the preoperative standing roentgenogram. We recommend that tibial osteotomy be done only when there is less than 10 degrees of varus deformity. In a knee with more than 15 degrees of varus deformity there will be subluxation on weight-bearing and tibial osteotomy is contraindicated.