Our purpose is to investigate the electrophysiological characteristics of Bell's palsy and to obtain clues for estimating prognosis in the late period by using electroneurography. Thirty-three patients were followed by electroneurography over a period of 12 months. They were classified according to House Brackman system. At the end of the follow-up, 100% of grade II-III patients, and 61% of grade IV patients recovered completely. Thirty per cent of grade IV patients recovered as grade II, and one grade IV (8%) and one grade V (100%) patient had bad prognoses (grade IV). There were significant differences between each group in the time course between the first and third months of onset. We concluded that the amount of non-degenerated synchronous fibres can allow us to estimate prognosis of Bell's palsy, especially between the first and third month of onset, if we make serial tests.