Dehiscence of the lateral semicircular canal (LSCD) has been reported much but mainly in association with cholesteatoma and canal wall down mastoidectomy, while idiopathic LSCD was rarely reported. Bassim reported one case with lateral semicircular canal dehiscence, but presented no vestibular or auditory symptoms. The patient in this study complained significant sound-induced vertigo and autophony in his right ear. The axis of nystagmus was orthogonal to the lateral semicircular canal, and no torsional or vertical motions were observed, so pathology of the lateral semicircular canal was preferentially considered. Benign paroxysmal positional vertigo was excluded since vertigo attacks had no relation to the change of head position. The dehiscence of the right lateral semicircular canal was then confirmed through the high-resolution temporal bone computer tomography scan and the reconstructed images. The cause of the LSCD is poorly understood, since no history of head trauma, otological infection or surgery was documented.