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Thoracolumbar kyphosis (TLK) is common in infants with achondroplasia. Our goals were to examine the natural history of TLK and identify factors associated with persistent TLK.We reviewed records of patients with achondroplasia seen by a board-certified orthopaedic surgeon at a tertiary care medical center between 1997 and 2013. Inclusion criteria were minimum 2-year follow-up and radiographs taken at time of presentation, within 6 months of walking age, and within 6 months of the first anniversary of walking age. We defined TLK as kyphosis of ≥20 degrees centered at T12 and L1. We assessed patient demographic characteristics, radiographic parameters (Cobb angle, apical vertebral translation, and apical vertebral wedging for vertebral height and width), and clinical parameters (developmental motor delay, hydrocephalus, presence of a ventriculoperitoneal shunt, and foramen magnum decompression). Developmental motor delay was defined as the inability to sit or ambulate independently by age 14 or 30 months, respectively. Associations between these factors and persistent TLK (ie, unresolved at final follow-up) were evaluated using logistic regression and χ2, Fisher exact, and independent t tests. Significance was set at P<0.05.A total of 60 patients were included. Mean values were as follows: age at presentation, 10.9±7.0 months; length of follow-up, 5.7±3.6 years; initial curve, 43.8±11.0 degrees; independent sitting age, 12.6±5.5 months; and independent walking age, 21.1±7.8 months. At walking age and 1 year after walking age, 15% and 58% of patients, respectively, had spontaneous TLK resolution. In total, 30% of patients had persistent TLK at final follow-up. Apical vertebral translation (P=0.001), percentage of apical vertebral wedging for vertebral height (P=0.031), and developmental motor delay (P=0.043) were associated with unresolved TLK.In patients with achondroplasia, TLK resolved at walking age in 15% of patients and after a year of walking in 58% of patients. Earlier bracing may slow TLK progression in patients with achondroplasia and developmental motor delay. Patients with kyphotic curves between 20 and 40 degrees should be examined intermittently for progressive deformity or worsening symptoms of spinal cord compression.Level II.