BACKGROUND Several studies have described the differences in electromyographic activity and histological changes of paravertebral muscles in patients with adolescent idiopathic scoliosis (AIS). However, there is little knowledge about the muscle volumetric and fatty infiltration imbalance of patients with AIS. MATERIAL AND METHODS Thirty-four patients with AIS were evaluated with standardized anteroposterior (AP) and lateral standing films for the location and direction of the apex of scoliosis, coronal Cobb angle, apex vertebra translation, and thoracic kyphosis; and with magnetic resonance imaging (MRI) scan of the spine at the level of T4-L1. The muscle volume and fatty infiltration rate of bilateral deep paravertebral muscles at the level of upper end, apex, and lower end vertebra were measured. RESULTS All patients had major thoracic curve with apex of curves on the right side. The muscle volume on the convex side was larger relative to the concave side at the three levels, while the fatty infiltration rate was significantly higher on the concave side. The difference index of the muscle volume was significantly larger at the apex vertebra level than at the upper end vertebra level (p=0.002) or lower end vertebra level (p<0.001). The difference index of muscle volume correlated with apex vertebra translation (r=-0.749, p=0.032), and the difference index of fatty involution correlated with apex vertebra translation (r=0.727, p=0.041) and Cobb angle (r=0.866, p=0.005). CONCLUSIONS Our findings demonstrated significant imbalance of muscle volume and fatty infiltration in deep paravertebral muscles of AIS patients. Moreover, these changes affected different vertebra levels, with the most imbalance of muscle volume at the apex vertebra. We interpreted this as morphological changes corresponding with known altered muscle function of AIS.