Objective: To investigate variations in the position of the cerebellar tonsils in outpatients with scoliosis of less than 50°, and to assess the correlation between tonsillar position and the progression of scoliosis. Study Design: A prospective study of magnetic resonance imaging findings in outpatients with idiopathic scoliosis. Summary of Background Data : Some patients with presumed idiopathic scoliosis have underlying cerebellar tonsillar herniation, such as the Chiari I malformation or tonsillar ectopia. With the development of magnetic resonance imaging (MRI), the detection of tonsillar herniation has increased. Materials and Methods: A total of 118 girls with a primary diagnosis of idiopathic scoliosis (Cobb angle less than 50° in a standing position) underwent MRI to examine the position of the cerebellar tonsils. Inferior displacement of the tonsils by ? 5 mm below the foramen magnum was defined as Chiari I malformation, and displacement by < 5 mm was defined as tonsillar ectopia. The definition of curve progression was increment of the Cobb angle by 6° or more on any two consecutive occasions. All patients were followed longitudinally until skeletal maturity or until curve progression occurred. The association between the position of the tonsils and progression of scoliosis was then analyzed. Results: The incidence of the Chiari malformation was 3.4 % (4/118 patients) and that of tonsillar ectopia was 8.5 % (10/ 118 patients). Two patients had a Chiari I malformation combined with syringomyelia. Patients with the Chiari malformation showed a significantly higher incidence of curve progression (p=0.006), but no significant correlation was found between tonsillar ectopia and curve progression ; however, tonsillar ectopia patients with displacement of > 2 mm below the foramen magnum showed a significantly higher incidence of curve progression (p=0.048). Conclusions: The Chiari I malformation might be associated with the progression of scoliosis. Furthermore, tonsillar ectopia with displacement of > 2mm below the foramen magnum is associated with a higher incidence of curve progression than ectopia with displacement of < 2mm. Based on these findings, tonsillar ectopia of > 2mm is the pathologic position of the cerebellar tonsils for the progression of scoliosis, and should be followed up as carefully as Chiari I malformation. It was previously reported that with growth of the skull bones, brain stem compression decreases, and scoliosis improves or stabilizes. With tonsillar ectopia or Chiari I malformation, the position of the tonsils might improve, and if the tonsils become less than 2mm below the foramen magnum, there is some possibility of improvement or stabilization of curve progression.