”Segmental duplication” (SD) of the basilar artery has a reported angiographic prevalence of 0.04% to 0.6%. It is presumed to occur as a result of maldevelopment during the fifth week of the fetal life, and should be differentiated from aortic dissection or embolism. A 72-year-old male presented with episodic positional dizziness for over one year. A magnetic resonance image/angiogram incidentally revealed a 1.74×0.84 mm vascular loop over the basilar artery, which confirmed a SD. We suggest that the SD increased the vascular resistance of the posterior circulation, which then led to transient unstable flow during postural hypotension. Conservative treatment and life-style changes were recommended. Over the following month, the symptom recurred less and less, and then medication was halted. Over the following year, the symptom only recurred at irregular intervals of several weeks or months. Thus it is possible that a simple life-style change may be able to prevent recurrence.