The spinal cords were examined in eighteen cases of multiple sclerosis, with special attention to the cervical enlargement. It was found that (1) lesions in the cervical cord are about twice as common as at lower levels, (2) in this region there is a striking preponderance of fan-shaped lesions in the lateral columns. It is argued that both these findings are explicable on the theory that mechanical stresses play a part in determining the site of lesions; that such stresses are commonly transmitted to the cord via the denticulate ligaments during flexion of the spine; and that many of the lesions are attributable to vascular leakages due to tension in the denticulate ligaments. It is concluded that in patients with multiple sclerosis neck flexion is dangerous--especially in cases where Lhermitte's sign has occurred.