ABSTRACT Objective: To create a new comprehensive, three-dimensional, applicable classification for Adolescent Idiopathic Scoliosis and similar conditions. Methods: The Three-Dimensional Classification was created with three components: the first, the quantitative factor, divides into three types according to the number of curves; the second, the locator factor defines the most structured point of the curve; and the third, the sagittal factor, evaluates the overall sagittal plane. To test the new classification, we studied the images of 99 patients comparing the intra- and interobserver agreement and reproducibility index of the Three-Dimensional Classification with that of Lenke. Results: It can be stated that, overall, the agreement between the three evaluators in relation to the Three-Dimensional Classification and that of Lenke in this series was considered very good. Conclusions: The case study showed a significant difference in the percentages between the two Classifications. In the evaluation of thoracic kyphosis, the Three-Dimensional Classification defined 26.6% of the cases as hyperkyphosis and 61.6% as normal, whereas the Lenke Classification defined 6.06% as hyperkyphosis and 84.18% as normal. However, in the global comparative analysis of the methods, the Three-Dimensional and Lenke systems presented statistically the same levels of agreement, since the values of the confidence intervals overlap. Level of Evidence II; Retrospective study.