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Adolescent idiopathic scoliosis (AIS) is a three-dimensional spine deformation with elusive aetiopathogenesis. One appealing hypothesis points to its neurologic origin with an emphasis on a vestibular impairment. In the present study, we explored the hypothesis of a vestibular deficit accompanying AIS by assessing differences in the subjective estimation of the gravitational vertical between adolescents with idiopathic scoliosis (n = 10, age 11–16 years, Cobb’s angle > 15°) and healthy age-matched controls (n = 10). Group participants actively controlled the verticality of a visual line in two visual conditions (eyes open-visual feedback and eyes closed-no visual feedback) and using three different segments (hand, head, and trunk). An electromagnetic tracking sensor (Nest of Birds, Ascension Ltd., USA, 60 Hz), attached either to a hand-held rod, the head, or the upper trunk, measured the line’s deviation from the gravitational vertical that was reflected in two measures, the mean absolute and variable error. The head’s medio-lateral tilt when estimating verticality with the hand was also registered. Analysis revealed that adolescents with idiopathic scoliosis made a greater error than control participants when estimating verticality with the head and eyes closed. In addition, they adopted a significantly greater head tilt when estimating the vertical by controlling the hand-held rod, regardless of the availability of vision. The error in the earth vertical was greater when the estimate was performed in the absence of vision. Results suggest a malfunction of the vestibular system and/or a sensorimotor integration impairment in patients with AIS, while vision compensates for the observed deficit in estimating the earth vertical.