Determining side of vestibular lesion remains a challenging task during neurotologic investigations, particularly if auditory lateralizing signs are absent. Traditionally, absent or decreased caloric response has been regarded as evidence of unilateral vestibular deficit. But, the inability of the chair test to detect side of lesion has been generally accepted because rotation stimulates both end organs. A retrospective study postulated and tested a hypothesis that low-frequency asymmetry of sinusoidal harmonic acceleration (SHA) testing can determine side of lesion when phase is abnormal. The present prospective, double-blind study was designed to further test the retrospective study hypothesis. Results showed that SHA asymmetry detects side of unilateral vestibular deficit when phase is abnormal in 97% of cases.