Bell's palsy has been reported as an adverse event following immunization (AEFI). Review of the published literature reveals that several characteristics have been used to describe Bell's palsy, which differ significantly from author to author. Evidently, the definition of "Bell's palsy" remains controversial, and consensus between different medical subspecialties is urgently needed. The Brighton Collaboration has formed an international working group with representatives of neurology, otorhinolaryngology, pediatrics, electrophysiology, pharmacology, pharmaceutical and biotech industry as well as regulatory agencies to create a case definition of Bell's palsy as an AEFI.