A 56-year-old man presented with a 3-month history of progressive dizziness. His dizziness was aggravated when his head was rotated to the right side. Diagnostic angiography showed that a normal right-sided vertebral artery in the neutral position became an abnormal vertebral artery with two stenotic lesions at the C3-4 and C5-6 levels when the patient's head was turned to the right. A normal left-sided vertebral artery also showed a stenotic lesion at the C2 level when the patient's head was turned right. The axial dimensions of the bilateral vertebral arteries were similar. The patient was successfully treated with decompression of only one level (C5-6). We conclude that if a bilateral stenosis is found upon one directional head rotation and the bilateral vertebral arteries are similarly sized, a one-sided treatment may suffice.