There is no data that shows if it is possible to determine if a curve is structural or non-structural or to assess flexibility of an adolescent idiopathic scoliosis (AIS) by magnetic resonance imaging (MRI) instead of bending radiographs (BR). We investigated if the results of BR may be compared to those of MRI. We retrospectively analyzed prospectively collected data of patients with AIS in whom a selective spinal fusion was performed and in whom a MRI, BR and full-spine X-rays were obtained preoperatively. We measured the Cobb angles of the main and of the minor curve in full-spine X-ray (FSR), BR and MRI and analyzed the degree of the intervertebral disk degeneration in the MRI. After applying inclusion and exclusion criteria, 25 patients were included. We found a significant correlation (p < 0.05, Corr Coeff = 0.41) between the Cobb angle of the main curve in FSR and the Cobb angle of the main curve in the MRI and between the Cobb angle of the minor curve in FSR and the Cobb angle of the minor curve in the MRI (p < 0.001, Corr Coeff = 0.04). All patients with a minor curve of less than 25° in the BR had a Cobb angle of less than 30° in the MRI. Spinal curves showed a significant correlation between bending radiographs and recumbent images (MRI). In our group of patients, a Cobb angle of the minor curve of less than 30° in the MRI indicated that this minor curve was non-structural according to the classification of Lenke.