Purpose : To evaluate normal tonsillar position at different ages, using MR images. Material and Methods : Wemeasured the position of the cerebellar tonsils with respect to the inferior aspect of the foramen magnum onmiddle sagittal MR T1 weight images in 210 patients(9 days to 83 years) who were not considered to have anydisorder that would affect tonsillar position. Results : The mean distance between tonsillar tip and inferioraspect of the foramen magnum was 0.0 $pm$ 1.9mm in the first decade of life, 0.7 $pm$ 2.8mm in the fourth decade, and3.4 $pm$ 2.3 mm in the eighth and ninth decades. The tonsillar position is significantly lower in youth(first andsecond decades) than in old age(seventh to ninth decades). Conclusion : We believe that a single referencestandard that indicates the normal distance of the cerebellar tonsils from the foramen magnum is inappropriateunless age is considered. We suggest that the following distance below the foramen magnum(more than 2 standarddeviations out of the normal range) be used as criteria for ectopia of the cerebellar tonsils in koreans : firstand second decades, 5mm ; third to sixth decades, 4mm ; seventh and eighth decades, 3mm.