Myelin basic protein (MBP) in the cerebrospinal fluid (CSF) was quantified from 28 patients with acute facial palsy. The mean value of MBP in CSF in the study group was 3.4±0.22 μg/1 (mean±SEM), which was significantly higher than the found in 37 healthy subjects (2.4±0.13 μg/1). Using a 95% confidence interval, 10 patients demonstrated significantly increased CSF concentrations of MBP (>3.95 μg/I). Auditory brainstem response (ABR) test showed that 5 patients had abnormal brainstem responses. No significant correlation was observed between elevated MBP in CSF and either pathologic auditory brainstem responses or the clinical degree of the palsy. These results suggest that a brainstem involvement may occur in patients with Bell's palsy. Furthermore, our data accord with previous reports suggesting Bell's palsy to be part of a polyneuropathy.