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Spondylothoracic dysplasia is a condition in which bilateral chest wall deformity due to costovertebral rib fusion with shortening of the thoracic spine results in severe thoracic insufficiency syndrome and early death. Little is known about the long-term respiratory natural history of this disorder and the specific anatomic deformity.We conducted a multicenter prospective and retrospective study of patients with spondylothoracic dysplasia. Medical evaluations, respiratory history, physical examination findings, radiographs, computed tomographic scans, and pulmonary function tests were studied. Anatomic, radiographic, and functional parameters for the disorder were established to determine the natural history of the thoracic insufficiency syndrome.Twenty-eight patients were identified. Eight patients had died in the neonatal period, and twenty were evaluated (eleven prospectively and nine retrospectively). The survivors were doing well clinically, but the average spirometric values were 27.9% of the predicted normal value for the forced vital capacity (FVC), 29.5% of the predicted normal value for the forced expiratory volume in the first second (FEV1), and 0.92 for the FEV1/FVC ratio, demonstrating a severe restrictive respiratory pattern. The computed tomographic scan lung volumes were an average of 28% of the expected values for age and gender. The thorax was stiff from rib fusion and was severely shortened posteriorly, averaging 24.2% of the predicted normal length. The thoracic spine was predominantly composed of block vertebrae, whereas in the lumbar region there were multiple hemivertebrae. Minimal scoliosis was seen, and there were no neurological deficits.Spondylothoracic dysplasia has a unique pathoanatomy of volume depletion deformity of the thorax with chest wall stiffness, resulting in thoracic insufficiency syndrome. Clinical tolerance of the restrictive lung disease in this disorder is impressive, but no clear reason has yet been identified for the clinical pulmonary health in the face of severe restrictive lung disease. Patients who survive infancy show no progression of congenital anomalies and can have a good quality of life. This disease may serve as a model of the natural history of thoracic insufficiency syndrome due to growth inhibition of the thoracic spine either as a result of congenital causes or secondary to surgical fusion early in life.Prognostic Level IV. See Instructions to Authors for a complete description of levels of evidence.