Historically, trunk movement has been thought to be reactionary to lower body motions. The excessive trunk movement in patients with myelomeningocele may be a primary mechanism during ambulation. The purpose of this study was to quantify three-dimensional trunk movement in patients with myelomeningocele. Patients walked with ankle-foot orthoses (AFOs), both with and without crutches. Data were collected in reference to global and pelvic coordinates. Patients who walked with AFOs and crutches showed less trunk dynamic range of motion than patients without crutches. Between the two patient groups, there was a significant difference in trunk obliquity but not in trunk tilt or rotation. There is a good correlation between maximum trunk obliquity and coronal plane valgus knee stress. Overall, coronal plane valgus knee stress is multifactorial and trunk motion and external tibial torsion are major contributors. However, when using trunk kinematics to describe dynamic motion, both global and pelvic coordinate systems are important reference frames.