The case is reported of a patient with progressive left hemiparesis due to vascular compression of the medulla oblongata. Metrizamide computerized tomography cisternography revealed that the left vertebral artery was compressing and distorting the left lateral surface of the medulla. Compression was surgically relieved and symptoms improved postoperatively. Neurological and symptomatic considerations are discussed in relation to the topographical anatomy of the lateral corticospinal tract.