In AIS patients, the cerebellar tonsil position was significantly lower in the upright than the supine position (-0.7+/-1.5 vs +1.2+/-1.7, p<0.00001) while in normal subjects, there was no change in cerebellar tonsil position between upright and supine positions (mean +2.1+/-1.7 vs +2.2+/-1.8, p = 0.93). AIS patients also had a greater degree of tonsillar excursion between upright and supine positions compared to normal subjects (mean -1.9 +/-1.6 vs -0.1+/-1.7, p<0.00001). More AIS patients tended to have tonsillar descent in the upright position rather than the supine position though this did not reach statistical significance (28% in supine position vs 48% in standing position, p=0.15). There is slightly better correlation between tonsillar level and Cobb angle in upright position (r = 0.45, p = 0.01) than those in supine position (r= 0.31, p=0.03) in AIS patients.