Sudden hearing loss in MS has usually been attributed to brain-stem involvement. We report a patient with sudden hearing loss and BAEP and MRI evidence of a lesion in the eighth nerve. A 28-year-old woman, affected with definite relapsing-remitting MS for 4 years, complained of paresthesias and right-sided hearing loss. BAEPs (previously always normal) showed on the right side: absence of peak I, peaks III and V were very delayed (5.66 msec and 7.46 msec) with normal III-V IPL. Gadolinium MRI showed an enhancing lesion in the right eighth nerve. The patient was treated with 6-methylprednisolone 500 mg i.v. for 6 days. BAEPs were repeated on days 3, 6, 9, 16, 23, 30, 37 and 44; MRI on days 6 and 23. The latencies of peaks III and V progressively reduced and normalized on day 23. A very delayed peak I (2.82 msec) was already detectable on day 6. IPLs were always normal. The MRI was unchanged on day 6, and showed a reduction of the lesion on day 24. The present study is the first combined BAEP and MRI demonstration of an eighth cranial nerve lesion causing sudden hearing loss during a bout in an MS patient.