The aims of this study were to determine the influence of external tibial torsion on the effectiveness of the ankle-foot orthoses (AFO) in children with lumbosacral myelomeningocele. Forty patients with normal tibial rotation and 18 patients with excessive external tibial torsion were evaluated with three-dimensional gait analysis at their comfortable walking speed. The group with normal tibial rotation showed significantly greater knee extension and lower mean extension moment compared with the group with external tibial torsion (p < 0.05). The posteriorly and laterally deviated ground-reaction force relative to the knee-flexion axis compromises the ability of this force to facilitate knee extension. Patients with torsional magnitudes >20 degrees demand close inspection as candidates for derotation osteotomy. The AFO will continue to stabilize the ankle-foot complex, but improved knee motion, knee-extensor activity, and ultimately walking efficiency may be compromised.