The effect of four to six months use of ankle-foot orthoses (AFOs) and balance training was determined for six children with cerebral palsy. The six consecutively referred patients had spastic diplegia or hemiplegia, with hyperextension of the knee, and had accurate voluntary control of the trunk and hips. Fixed AFOs were adjusted to control the position of the ground reaction force in relation to the knees. Balance training was targeted to the knees. All children showed a decrease in the magnitude of the knee-extending moment arm toward normal when barefoot. Improvement was noted in foot-ground contact for three children and of stance-phase posture for three. These improvements were not related to range of motion or speed, and a motor learning effect is proposed. The value of monitoring the moment arm is emphasised.