A 70-year-old man presented with acute dysarthria and dizziness. He denied any history of trauma or cervical manipulations within several weeks before symptom onset. We could make a presumptive diagnosis of left Wallenberg syndrome through the results of neurologic examination, which include left limb ataxia, alternating hyp(o)esthesia, spontaneous nystagmus to right side, and left side Honer's syndrome. Initial diffusion weighted imaging performed at admission showed small and discrete high signal lesions in left lateral medulla, left cerebellar hemisphere, and bilateral occipital areas. Contrast enhanced MRA demonstrated a filling defect in long segment of distal left vertebral artery. On 4th days after symptom onset, the patient developed a severe form of ipsilateral hemiparesis. Follow-up brain MRI showed a downward extension of the initial ischemic lesion in upper medulla to upper cervical region. This case suggests that a severe form of ipsilateral hemiparesis may be complicated in the clinical setting of acute lateral medullary infarction with vertebral artery occlusion.