Introduction The sagittal profile of conventionally and surgically treated scoliotic spines is usually analyzed via lateral views of whole-spine X-rays in an upright position. Due to a more hypokyphotic configuration of scoliotic spines, the view onto the upper thoracic vertebrae is often difficult. We investigated whether additional supine MRI measurement supports valid kyphosis angle measurement. Patients and methods Twenty patients with either short (n = 10, Halm-Zielke, VDS) or long spondylodesis (n = 10, dorsoventral) were assessed 5 years after surgery with standing radiographs and supine whole-spine MRI. Results Up to 90% of the upper thoracic vertebrae were invisible on radiographs, whereas MRI allowed visibility of almost many vertebrae. No significant difference in thoracal kyphosis angles could be observed in the comparison of X-ray and MRI data. Conclusion Thoracal kyphosis measurement of postoperative spines in MRI is a valid diagnostic tool with reliability comparable to that of X-ray. These results cannot be transferred to lumbar lordosis measurement and transferred only partly to coronal COBB angle measurement.