A total of 222 adult subjects, all of whom received primary immunization according to the rapid immunization schedule in a preceding clinical trial with either a new (i.e. polygeline free) or formerly licensed (i.e. polygeline containing) TBE vaccine were invited for extension studies. The subjects received the first booster immunization with the new TBE vaccine at 12 to 18 months after primary immunization. Subsequently, a total of 191 of the 222 subjects could be enrolled in a serological follow-up one year after the booster immunization. Neutralizing TBE antibody titers were determined prior to, 21 days after and approximately 12 months after booster immunization. Prior to first booster immunization, TBE antibodies (GMTs) had remained on a high level and were far above the detection limit of the neutralization test used. All subjects of the per protocol population who were primarily immunized with the new TBE vaccine formulation and all but one subject of the control group were still seropositive prior to the booster. All subjects showed a sharp increase of TBE antibodies following the booster immunization. Within the 12 months follow-up period, neutralizing TBE antibody titers remained on a high level. The booster vaccination was well tolerated by the subjects. Only very few febrile reactions (< 1%) none higher than 38.5 degrees C were reported. No serious or unexpected adverse events related to vaccination were reported. These successful results in terms of both immunogenicity and safety indicate that TBE vaccination with this new TBE vaccine can be used safely in adults. A long lasting immunity can be concluded from the strong immune response following the booster immunization.