Clinical presentation of multiple sclerosis (MS) is highly heterogeneous. Deafness is considered to be rare. The aim of this study was to determine the frequency of acute deafness in MS and to evaluate prognosis. The occurrence of acute deafness was systematically noted in a cohort of 400 consecutive clinically definite MS patients. Acute deafness was observed in 14 patients (3.5 p. cent) with a mean age of 28 years (15 to 48) and after a mean of 2.3 years (0 to 10) of disease duration. In all but two patients, deafness was associated with disease exacerbations, highly suggestive of infratentorial lesions. MRI showed hyperintensities in the pons or the midbrain in 11 cases. Deafness disappeared in all cases excepted for one patient. After a mean follow-up of 7.2 years (1 to 17), all the patients remain in the relapsing-remitting form of the disease with a mean EDSS of 2.4 (0 to 4.5). Acute deafness is observed in 3.5 p. cent of MS cases and occurs more frequently during the first years of the disease. Although follow-up of a larger cohort is necessary, deafness could be of good prognostic factor in MS.