We defined sudden vertigo as a sudden, unilateral peripheral vestibular dysfunction. The criterion for its diagnosis is a single episode of vertigo without cochlear and central symptoms. Among 20 patients with sudden vertigo there was no difference in clinical aspects between those with CP (canal paresis) (CP%≥25%) and those without CP (CP%<25%). This suggests that sudden vertigo with CP is due to sudden vestibular dysfunction with predominant involvement of the lateral semicircular canal. Basically, vestibular neuronitis is considered to be due to acute unilateral neuropathy of the vestibular nerve. However, since we have no routine examination for evaluating vestibular nerve function, sudden vertigo with CP should be diagnosed as vestibular neuronitis. We then assessed the prognosis of sudden vertigo with CP (vestibular neuronitis). About two years after the onset of CP 4 of 10 patients had recovered. However, patients with persistent CP had a handicap in their everyday life because of the dizziness induced by head movements. The possibility of recovery of vestibular function in response to steroid therapy may improve the prognosis in vestibular neuronitis.