In order to evaluate with accuracy the tridimensional (3D) vertebral correction induced by the surgical correction of idiopathic scoliosis with Cotrel-Dubousset instrumentation and technique, we have developed a new per operative measurement system based on 3D digitization with magnetic fields. This method has been used on 23 adolescent patients treated with surgical correction. A statistically significant change in vertebral 3D orientation of 11.4 degrees +/- 5.6 degrees has been found. The measurement repeatability was +/- 2.5 degrees. We conclude that the Cotrel-Dubousset technique truly realizes a 3D correction of the thoracic and lumbar spine of adolescent idiopathic scoliosis.