The spinal canals of 11 patients with various pathological conditions, both benign and malignant, were examined intraoperatively using real-time ultrasonography. The spinal cords in eight of these patients with lesions causing cord compression were seen to oscillate rhythmically over each mass with a frequency corresponding to the patient's arterial pulsations. The majority of the observed motion was transmitted or extrinsically induced from compression of the anterior spinal artery. In one case, after resection of an anterior cervical neurofibroma, these oscillations diminished markedly. Spinal cord motion, as described, has important implications for neurosurgery, since it contradicts the previously held belief that oscillations of the spinal cord or dura mater imply a "free" or noncompressed spinal cord. Furthermore, spinal cord motion may play a role in the degradation of computerized tomography images of the cord, since these movements are maximized at the areas with the most severe pathology.