The purpose of this study was to determine in vitro the centers of rotation of thoracic functional spinal units in the sagittal plane. The center of rotation is a convenient concept and part of a precise method of documenting the kinematics of a joint moving in a plane. Fresh cadaver functional spinal units from the thoracic region were utilized. Six load types were used that produced motions only in the sagittal plane, namely anterior and posterior shear forces, flexion and extension moments, and compression and distraction forces. The resulting motion with three degrees of freedom was measured with dial gauges. Statistical methods were used to analyze data from the viewpoint of vertebral level, load magnitude, and load type. Only the load type was found to be significantly related to the location of the centers of rotation. Although there was significant variability in the centers of rotation, there were definite locations related to each load type. The average center of rotation was 15-45 mm directly below the geometric center of the moving vertebra. The results of the present study may be helpful in the clinical interpretation of spinal kinematic studies.