Trigeminal sensory evoked potentials (TSEP) were determined in Bell's palsy cases of recent onset, and the following findings were observed: 1. TSEP was recorded with an exploring electrode at C5 or C6 on the scalp on the side opposite the Bell's palsy and with a reference electrode at Fz by stimulation with a bipolar stimulating electrode at the middle point of the inside of the upper lip. A tetra-phasic (N12, P20, N29 and P41) stable normal wave pattern was achieved. 2. TSEP was evaluated from the right-left difference for each case on the basis of a decrease (by less than 50%) in amplitude and an increase (by more than 10%) in the latency. 3. Ten of 20 patients with Bell's palsy showed abnormal TSEP. 4. Attempts at evaluation of the severity, course and prognosis of facial palsy by TSEP were unsuccessful. 5. All of the patients with an antibody titer for herpes simplex virus (HSV) of less than 4-fold showed normal TSEP. 6. The majority of patients with abnormal TSEP were considered to have been suffering from HSV infection. 7. Some patients with HSV infection showed normal TSEP. 8. Taking into consideration that patients with abnormal TSEP were infected with HSV, acyclovir was administered.