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Purpose: To assess the dosimetric effects of respiratory motion on the target and spinal cord in spinal stereotactic body radiation therapy (SBRT)., Methods and Materials: Thirty patients with 33 lesions were enrolled on a prospective clinical protocol and simulated with both free-breathing and four-dimensional (4-D) computed tomography (CT). We studied the target motion using 4-D data (10 phases) by registering a secondary image dataset (phase 1 to 9) to a primary image dataset (phase 0) and analyzing the displacement in both translational and rotational directions. The study of dosimetric impacts from respiration includes both the effect of potential target and spinal cord motion and anatomic changes in the beam path. A clinical step-and-shoot IMRT plan generated on the free-breathing CT was copied to the 4-D datasets to evaluate the difference in the dose-volume histogram of target and normal tissues in each phase of a breathing cycle., Results: Twenty three lesions had no motion in a breathing cycle; four lesions had anterior-posterior motion ≤ 0.2 mm; two lesions had lateral motion ≤ 0.2 mm; and eight lesions had superior-inferior motion, most ≤ 0.2 mm with the worst at 0.6 mm. The difference of maximum dose to 0.01 cm 3 of spinal cord in different phases of a breathing cycle was within 20 cGy in worst case. Target volumes that received the prescription dose (V100) varied little, with deviations of V100 of each phase from the average CT < 1% in most cases. Only when lesions were close to the diaphragm (e.g., at T11) did the V100 deviate by about 7% in the worst case scenario. However, this was caused by a small dose difference of 20 cGy to part of the target volume., Conclusions: Breathing induced target and spinal cord motion is negligible compared with other setup uncertainties. Dose calculation using averaged or free-breathing CT is reliable when posterior beams are used., Competing Interests: Authors’ disclosure of potential conflicts of interest The authors reported no conflict of interest.