A study was undertaken to establish the significance of the sagittal shape of the spine in 138 consecutively treated girls with early idiopathic thoracic scoliosis. All were treated in a Boston brace. Two groups were formed: group I consisted of 120 girls whose scoliosis remained stable in the brace, group II of 18 girls who required a spinal fusion for progressive curves despite the brace. For each spine, the orientation of each vertebra to the horizontal in the sagittal plane was measured on the earliest lateral radiogram, taken when the anterior curve still had a Cobb angle of less than 20 degrees. A significant difference in spinal profile was found between the two groups. Progressive curves showed a more retroverted orientation of mid- and high thoracic vertebrae than stable curves.