This study is an investigation into the position of the cerebellar tonsils in AIS with severe curves and any relationship it has with age, sex, curve severity and curve pattern. Sagittal magnetic resonance imaging (MRI) of hindbrain was performed on both of 205 AIS patients with a Cobb angle greater than 40 degrees and 86 healthy controls. The position of the cerebellar tonsil relative to the line connecting the basion and opisthion (BO line) was measured. In AIS and controls, the mean position of the cerebellar tonsil was 0.9 and 2.9 mm above the BO line, respectively. The incidence of tonsillar ectopia in AIS was found to be significantly higher than controls (34.5% versus 5.8%, p<0.001). No significant correlations were found between the position of the cerebellar tonsil with age or sex in AIS and controls. It was shown the position of the cerebellar tonsil was not significantly different among AIS patients with deferent curve severity. However, a significant lower incidence of tonsillar ectopia (p=0.049) was found in patients with lumbar curves when compared to those with thoracic or thoracolumbar curves. In conclusion, there was a relatively lower position of the cerebellar tonsil together with a significant higher incidence of tonsil ectopia in AIS patients. There was a trend that tonsillar ectopia was more often in thoracic or thoraco-lumbar curves, suggesting that a lower position of the cerebellar tonsil may play an important role in the etiopathogenesis of AIS.