Study design: Reproducibility study. Objectives: Assess the agreement and reliability of the classification of early onset scoliosis (C-EOS). Summary of background data: C-EOS is a promising tool for patients with early onset scoliosis (EOS). However, the reliability has only been examined without measuring radiographs and not including the annual progression rate (APR) modifier. Methods: We included a single-center consecutive cohort of patients diagnosed with EOS seen in our outpatient clinic. Patients had no previous spine surgery. Four raters rated 60 cases. Two anterior–posterior full-spine radiographs, taken minimum 6 months apart, and one sagittal radiograph were measured twice by all raters in a blinded test–retest setup. Results were assessed using crude frequency of overall agreement (OA), intra- and inter-rater Fleiss kappa (κ) statistics, and intraclass correlation coefficient (ICC). We calculated the 95% limits of agreement (LOA) for major curve angle (MCA), kyphosis, and APR using a linear mixed-effects model. Inter- and intra-rater LOA were analyzed for each etiology separately. Results: Mean age was 8.7 ± 3.4 years and the etiology were congenital/structural (n = 20), idiopathic (n = 19), neuromuscular (n = 13), or syndromic (n = 8). For etiology, OA was 75.8% and κ = 0.80. For major curve angle, OA was 84.2%, κ = 0.86, ICC = 0.97, and LOA = 12.8°. For kyphosis, OA was 55.8%, κ = 0.52, ICC = 0.87, and LOA = 20.6°. For APR, OA was 76.7%, κ = 0.61, ICC = 0.77, and LOA = 17.4°/year. Inter- and intra-rater LOA were generally largest for neuromuscular and smallest for idiopathic patients. Conclusions: We found substantial agreement for etiology, however, with disagreement in certain cases. The reliability of MCA was excellent; however, somewhat lower for kyphosis and APR with less accuracy. The measurement errors of MCA, kyphosis, and APR depended largely on the etiology. Regarding APR, LOA exceeded the 10°/year increments proposed in the C-EOS, suggesting a revision of this optional modifier. Level of evidence: Diagnostic study level 1.