The prone operative position affects the geometry of the scoliotic spine. However, the literature provides little information concerning the effect of operative position on the scoliotic trunk geometry. This study compared the trunk deformity of 31 adolescent idiopathic scoliosis patients in two standardized positions: standing and prone on a Relton-Hall frame. The geometry of the trunk was assessed using 11 magnetic sensors placed on anatomical landmarks. Eight geometric indices of the trunk were compared using paired Student t-tests (alpha=0.01). The transverse rotation of shoulders, the coronal rotation of shoulders, the lateral shift of C7 with respect to S1, and the thoracic and lumbar angles of trunk rotation assessed in the operative position did not show any significant change compared to the standing position. Therefore, operative positioning using the Relton-Hall frame did not lead to an overall reduction in the scoliotic trunk deformity. Modifications to the Relton-Hall frame are required for optimal presurgical correction of the scoliotic trunk deformity.