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To determine the incidence and characteristics of hyperventilation-induced nystagmus (HIN) in cerebellopontine angle (CPA) tumors and unilateral peripheral vestibulopathy (UPV), and to elucidate differential contribution of hyperventilation to bring out vestibular asymmetry between acute and chronic phases of UPV.We recorded horizontal HIN in 33 patients with CPA tumors and 145 with UPV. The UPV included patients of either acute (7 days or less from symptom onset, n = 47) or chronic (more than 7 days from symptom onset, n = 98) phases.The incidence of HIN was higher in the CPA tumor than in the UPV group (82 vs 34%, p0.01) and was also higher in the acute than in the chronic UPV group (60 vs 21%, p0.01). Furthermore, HIN was more commonly ipsilesional (i-HIN) in the CPA tumor than in the UPV group (52 vs 8%, p0.01) and more commonly ipsilesional in the acute than in the chronic UPV group (21 vs 1%, p0.01). The patients with i-HIN and acoustic neuroma had a tendency to harbor smaller tumors and to have less severe caloric asymmetry.The contribution of hyperventilation on vestibular nystagmus differs depending on the disease phase or underlying pathologies. Our study demonstrates that hyperventilation-induced nystagmus (HIN) beating to the side of reduced caloric response, hearing impairment, or abnormal auditory brainstem response responses may be a valuable sign for bedside detection of cerebellopontine angle (CPA) tumors. CPA tumor should be a prime suspicion in patients with acute vertigo and ipsilesional HIN, especially when the vertigo accompanies hearing impairments.