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Background: While early spinal fusion may halt progressive deformity in young children with scoliosis, it does not facilitate lung growth and, in certain children, it can result in thoracic insufficiency syndrome. The purpose of this study was to determine pulmonary function at intermediate-term follow-up in patients with scoliosis who underwent thoracic fusion before the age of nine years.Methods: Patients who had thoracic spine fusions before the age of nine years with a minimum five-year follow-up underwent pulmonary function testing. Forced vital capacity, forced expiratory volume in one second, and maximum inspiratory pressure were measured and compared with age-matched normal values. Patients with neuromuscular disease, skeletal dysplasias, or preexisting pulmonary disease were excluded, while those with rib malformations were included. The relationships between forced vital capacity and age at the time of surgery, length of follow-up, extent of the fusion, proximal level of the fusion, and revision surgery were studied.Results: Twenty-eight patients underwent evaluation. Twenty patients had congenital scoliosis, three had idiopathic scoliosis, three had scoliosis associated with neurofibromatosis, one had congenital kyphosis, and one had syndromic scoliosis. Seventeen patients had one spinal surgery, while eleven had additional procedures. The average age of the patients was 3.3 years at the time of surgery and 14.6 years at the time of follow-up. The average extent of the thoracic spine fused was 58.7%. The average forced vital capacity was 57.8% of age-matched normal values, and the average forced expiratory volume in one second was 54.7%. The forced vital capacity was <50% of normal in twelve of the twenty-eight patients, and two required respiratory support, implying that substantial restrictive lung disease was present. With the numbers studied, no significant correlation could be detected between the age at the time of fusion or the length of follow-up and pulmonary function. The extent of the spine fused correlated with the forced vital capacity (p = 0.01, r = −0.46). Fusions in the proximal aspect of the spine were found to be associated with diminished pulmonary function as eight of twelve patients with a proximal fusion level of T1 or T2 had a forced vital capacity of <50%, but only four of sixteen patients with a fusion beginning caudad to T2 had a forced vital capacity of <50% (p = 0.0004, r = 0.62).Conclusions: Patients with proximal thoracic deformity who require fusion of more than four segments, especially those with rib anomalies, are at the highest risk for the development of restrictive pulmonary disease. Pulmonary function tests should be performed for all patients who have an early fusion. The pursuit of alternative procedures to treat early spinal deformity is merited.Level of Evidence: Therapeutic Level IV. See Instructions to Authors for a complete description of levels of evidence.