Most research on equine kinematics has previously been performed in the walking and/or trotting animal. This is also true for the few studies on the kinematics of the equine back. These studies have, for the major part, focused on the flexion-extension movement in the sagittal plane. However, vertebrae can rotate in 3 dimensions. This study was designed to determine all 3 rotations in various segments of the vertebral column of a cantering horse. Five Dutch Warmblood horses were measured during treadmill canter (7.3 m/s). Steinmann pins were inserted into the dorsal spinous processes of 8 thoracic (T), lumbar (L) and sacral (S) vertebrae and into both tubera coxae. A set of 4 markers was rigidly attached to each pin. The marker data were used to calculate a rotation matrix that was subsequently decomposed into 3 orthogonal rotations (flexion/extension [FE]; lateral bending [LB] and axial rotation [AR]). For the 3 rotations the variability between the horses was low for FE, slightly larger for AR and largest for LB. The maximal range of motion (mean +/- s.d.) for FE, LB and AR was 15.8 +/- 1.3 degrees, 5.2 +/- 0.7 degrees and 7.8 +/- 1.2 degrees, respectively. With respect to relative angles, the largest FE motion was found between L5 and S3 with values for the range of motion up to 8.6 degrees. Simultaneous rotation of successive vertebrae was observed particularly during the single support and suspension phases in the stride cycle, which increases spinal stability. For all rotations, a close correlation was observed between the timing of the vertebral rotations and the pro- and retraction of the limbs.