The possible involvement of depression on cellular immunity in reactivation of varicella-zoster virus (VZV) in herpes zoster oticus was investigated. The subjects comprised 59 cases of herpes zoster oticus, 33 cases of herpes zoster sine herpete (ZSH) with facial paralysis, and 205 cases of Bell's palsy. The transformation rate of lymphocytes to phytohaemagglutinin in herpes zoster oticus tended to be lower than that in Bell's palsy. In skin tests with purified protein derivatives of tuberculin, the positivity rate in herpes zoster oticus was significantly lower than that in Bell's palsy (p < 0.015). In skin tests using VZV antigen the positivity rate in herpes zoster oticus and ZSH were significantly lower than that of Bell's palsy (p < 0.001 and p < 0.015, respectively). Thus, it was noted that cellular immunity, especially specific cellular immunity against VZV, was significantly depressed in herpes zoster oticus and ZSH. We consider that depression of specific cellular immunity plays an important role in triggering reactivation of VZV and onset of these diseases.