To elucidate mechanisms responsible for susceptibility of the palatine tonsil to infections in patients with recurrent tonsillitis, we studied 46 patients with recurrent tonsillitis (RT), simple hypertrophic tonsil (SHT) or focal tonsillitis (FT) in regard to the participation of viral infections, especially Epstein-Barr virus (EBV) infection by using ELISA and immunostaining methods. Culture supernatants of tonsillar lymphocytes in RT group had antibodies to respiratory syncytial (RS) virus at greater positive rate (26.3%) than that of SHT group (16.7%). About 70% of patients with RT had serum antibodies to viral capsid antigen (VCA) of EBV in high titre over 160X, but only 30% in SHT group. Furthermore, in RT group 2 patients possessed IgM antibody to VCA and IgG antibody to early antigen (EA). In RT group, IgG, IgM and IgA antibodies to VCA and IgG antibody to EA presented in the supernatant in 58%, 17.6%, 11.7% and 17.6% respectively. In the 35 tonsillar lymphocyte suspensions tested 20% contained EBNA-positive lymphocytes. In RT group, 5 of 21 patients (23.8%) had EBNA-positive lymphocytes, but only one of 9 (14.3%) in SHT group. These results suggest that EBV infection and its reactivation in the palatine tonsil may be one of major mechanisms in recurrent episodes of tonsillitis.