Both the humans as well as the quadrupedal spine have been shown to exhibit a pattern of pre-existent rotation that is similar in direction to what is found in the most common types of idiopathic scoliosis. It has been postulated that human bipedalism introduces forces to the spine that increase a tendency of the vertebrae to rotate. The objective of this study was to examine the effect of body position on vertebral rotation in vivo. Thirty asymptomatic volunteers underwent magnetic resonance imaging scanning of the spine (T2-L5) in three different body positions; upright, quadrupedal-like (on hands-and-knees) and supine. Vertebral rotation in the local transverse plane was measured according to a pre-established method and compared at different spinal levels between the three body positions. It was shown that in all three positions the mid- and lower thoracic vertebrae were predominantly rotated to the right. However, vertebral rotation was significantly less in the quadrupedal position than in both the standing upright and supine positions.