Electrophysiologic investigations were carried out on 45 patients with Bell's palsy at periodic intervals after the onset of paralysis. It was found that there was a good correlation between prognosis in Bell's palsy and the amplitude of evoked motor response obtained after six or more days of clinical paresis. When the average amplitude of evoked motor response was within normal limits (i.e., 504 mu V or greater), complete recovery with no residual deficits took place two to six weeks after the onset of facial palsy. When the evoked motor response was absent in all three major branches of the facial nerve, indicating complete nerve degeneration, electromyographic signs of recovery were apparent by the third of fourth month after the onset of paralysis. In these cases, recovery was relatively slow and incomplete, with some degree of residual deficit and synkinesis. Maximal return of voluntary facial movement was established 8 to 12 months after the initial symptom. When the mean amplitude of evoked motor response was below the lower limit of normal (i.e., less than 504 mu V), electromyographic signs of recovery were noted within 1 to 3 months, depending on the amplitude values. The final outcome of this intermediate group was similar, but not identical, to that of the previous group. The prognosis of facial paralysis in Bell's palsy was thus found to be directly related to the mean amplitude of evoked motor response, regardless of the extent of clinical paralysis.