An 84-year-old male with a 3-month history of headache and elevated C-reactive protein levels was admitted for biopsy of the superficial temporal artery, which led to the diagnosis of giant cell arteritis (GCA). Two days after prednisolone therapy was initiated, the patient began to experience transient vertigo attacks. Two days later, dysarthria, left-sided hemiparesis, right abducens palsy, and horizontal nystagmus developed. Magnetic resonance (MR) imaging disclosed fresh infarctions in the vertebrobasilar territory. Since the patient became drowsy because of brainstem compression and hydrocephalus due to cerebellar swelling, emergency suboccipital decompression surgery and ventricular drainage were performed. Subsequently, the patient's consciousness levels improved. MR angiography revealed right vertebral artery (VA) occlusion and left VA stenosis due to arteritis. Ischemic stroke is a serious though relatively rare complication of GCA. Similar cases have been reported, in which ischemic stroke developed despite or possibly due to steroid therapy. To our knowledge, this is the first description of vertebrobasilar infarction associated with GCA in the Japanese population. The merits and potential demerits of steroid therapy are briefly discussed.