['Retrospective analysis.', 'To determine if statistically significant differences exist between operative idiopathic early-onset scoliosis (IEOS) and adolescent idiopathic scoliosis (AIS) in primary curve characteristics, stable vertebra, kyphosis, or lumbar deformity and, if so, to identify the clinical significance of these differences.', 'To our knowledge, no study has statistically compared radiographical measures of operative IEOS and AIS.', 'We identified operative patients (60, IEOS; 1537, AIS) in 2 multicenter databases and measured preoperative radiographical parameters of interest. The measurements were compared using the Student t test and other appropriate statistical methods (significance, P = 0.05).', 'The IEOS and AIS groups were significantly different in primary curve magnitude (70° ± 20° vs. 54° ± 13°, respectively; P < 0.001), stable vertebra location (L3.0 ± 1.4 vs. L2.1 ± 2.2, respectively; P = 0.001), and T2 to T12 kyphosis (40° ± 15° vs. 31° ± 13°, respectively; P < 0.001). Distribution of major curve apex was unimodal centered close to the thoracolumbar junction in IEOS versus bimodal in AIS. Primary curve type was thoracic in 83% and 79% and thoracolumbar/lumbar in 17% and 21% of patients with IEOS and AIS, respectively. When the overall cohorts were separated into curve types and these subsets were compared, statistically significant differences were found between IEOS and AIS in thoracic primary curves (apex, magnitude, direction, proximal and distal Cobb vertebrae, and lumbar deformity) and thoracolumbar/lumbar primary curves (magnitude).'] Do clinically significant differences exist between curves in operative idiopathic early-onset scoliosis and adolescent idiopathic scoliosis?