Purpose To determine the association between coronal Cobb's angle and Nash-Moe index in patients with adolescent idiopathic scoliosis. We also attempted to determine whether apical vertebral derotation depended upon the curve flexibility. Overview of literature The three-dimensional nature of adolescent idiopathic scoliosis (AIS) is well established. Knowledge of all components of this complex deformity is essential to formulate effective treatment strategies. Though the importance of quantifying all the components of the deformity, in AIS, has been analysed in detail, very few studies have been done to ascertain the relationship between the coronal plane deformity and apical vertebral rotation. Methods Digitalised standing and supine stretch anteroposterior (AP) radiographs of 158 patients with AIS were analysed. The standing and supine stretch AP radiographs were compared to calculate the percentage reduction of Cobb's angle to determine curve flexibility. The derotation of the apical vertebra on application of traction was also noted. The one-way repeated ANOVA was used to determine the association between Cobb's angle and Nash-Moe index. The independent sample t test was used to determine whether a statistically significant difference was present, in the age of the patient, severity of the curve and percentage reduction of Cobb's angle between those curves that derotated and those that did not, when stretched. Results The one-way repeated ANOVA revealed an association between Cobb's angle and Nash-Moe index on the standing and supine AP stretch radiographs (P < 0.01). The Independent sample t-test showed a statistically significant difference in percentage reduction of Cobb's angle between those curves that derotated compared to those that did not, on stretch (P < 0.01). Conclusions This study demonstrates that there is an association between apical vertebral rotation and the coronal plane deformity. It also demonstrates that flexible curves derotate to a greater extent compared to rigid curves, when stretched.