Five and 6 years post-booster, immunity to tick-borne encephalitis (TBE) virus was assessed in 225 and 195 vaccinees, respectively, out of 430 healthy volunteers with at least three TBE-immunizations prior to study inclusion and booster intervals exceeding recommended limits. Neutralizing antibody titers of > or = 1:10 (reliable level of protection) were present in 86-96% depending on age group, with lower percentages in participants >60 years. TBE antibody levels remained stable for many years in most vaccinees. However, in a few persons a shorter period of protection against TBE was indicated. Therefore, recommendations on booster intervals in TBE endemic areas should be adapted by weighting the risk of infection against the risk of short-lived immunity.