OBJECTIVE This study set out to evaluate the hearing changes that occur during intratympanic gentamicin therapy and to correlate them with the long-term effects of the treatment on the control of vertigo and on hearing. METHODS This was a prospective study. METHODS Tertiary medical center. METHODS The 71 patients included in the study had been diagnosed with unilateral Ménière's Disease as defined within the 1995 American Academy of Otolaryngology-Head and Neck Surgery guidelines, and had been refractory to medical treatment for at least 1 year. METHODS Intratympanic injections of gentamicin at a concentration of 27 mg/ml were performed at weekly intervals until indications of vestibular hypofunction appeared in the treated ear. If there was a recurrence of the episodes of vertigo, an additional course of injections was performed. METHODS The 1995 American Academy of Otolaryngology-Head and Neck Surgery criteria for reporting the treatment outcome for Ménière's Disease were used. During the period of gentamicin instillation, weekly audiograms were obtained. The results of the treatment were expressed in terms of control of vertigo and hearing level. RESULTS Vertigo was controlled by gentamicin instillation in 83.1% of the 71 patients. Two years after the treatment, hearing loss as a result of the gentamicin injections was observed in only 11 (15.5%) patients. The recurrence of spells of vertigo after having initially achieved complete control was noted in 17 (23.9%) patients. Hearing loss at the end of the treatment occurred in 32.4% of the patients, but it was transitory so that 3 months after ending the treatment it was 12.7% and after 2 years it was 15.5%. Those patients in whom no change in their level of hearing occurred during the treatment needed another course of injections and presented poorer overall control of vertigo. CONCLUSIONS Ending weekly intratympanic injections when clinical signs of vestibular deafferentation appear results in the control of vertigo in the majority of patients. The hearing changes detected during the treatment are transitory and are the only clinical sign that predicts the response to gentamicin instillation.