When doctors evaluate the complaints of dizziness, they often perform a series of clinical tests to look for the evidence of a vestibular dysfunction. A useful procedure is to ask the patient to take deep breaths and observe the gaze behind Frenzel goggles. If hyperventilation-induced nystagmus(HIN) is detected, it is the evidence for an underlying vestibular imbalance. The authors evaluated nystagmus with electronystagmography after hyperventilation for 50 seconds. Brain imaging was performed to search the responsible lesion for dizziness. Brain MRI revealed a brain tumor suggesting vestibular schwannoma in the left cerebellopontine angle. After hyperventilation, dizziness and the right beating horizontal nystagmus with Alexander law could be detected. By precisely measuring the HIN, we determined that inputs arising from the horizontal semicircular canal were mainly responsible. The contralaterality of the direction of the horizontal component of the nystagmus was detected. We suggest that clinicians should routinely check the nystagmus after hyperventilation, when they evaluate patients complaining of dizziness.