PURPOSE: To evaluate normal tonsillar position at different ages, using MR images. MATERIAL AND METHODS: We measured the position of the cerebellar tonsils with respect to the inferior aspect of the foramen magnum on middle sagittal MR T1 weight images in 210 patients(9 days to 83 years) who were not considered to have any disorder that would affect tonsillar position. RESULTS: The mean distance between tonsillar tip and inferior aspect of the foramen magnum was 0.0 ±1.9mm in the first decade of life, 0.7 ±2.8mm in the fourth decade, and 3.4 ±2.3 mm in the eighth and ninth decades. The tonsillar position is significantly lower in youth(first and second decades) than in old age(seventh to ninth decades). CONCLUSION: We believe that a single reference standard that indicates the normal distance of the cerebellar tonsils from the foramen magnum is inappropriate unless age is considered. We suggest that the following distance below the foramen magnum(more than 2 standard deviations 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.