Background. In 2002, 2 cases of tick-borne encephalitis were diagnosed among inhabitants living in a tick endemic area on the island of Aspö in south-eastern Sweden. During the previous 25 years, only 2 additional cases had been diagnosed in that region of Sweden. To study presence and evolution of seroprevalence of antibodies to the tick-borne encephalitis virus we carried out a follow-up study, comparing inhabitants´ immunoglobulin G antibody levels against the virus in blood samples drawn in 1991 and 2002. Method. The island of Aspö is located in the south-eastern archipelago by the Baltic Sea in the county of Blekinge, Sweden. Due to the confirmed cases of tick-borne encephalitis, permanent and part-time residents were offered tick-borne encephalitis vaccination in the autumn of 2002. Blood samples were collected and analyzed by the two-step enzyme-linked immunosorbent assay to detect immunoglobulin G antibodies against tick-borne encephalitis virus. Also, questionnaires including questions about sex, age, earlier history of and previous vaccination against tick-borne encephalitis, residency on Aspö, history of observed tick-bites and earlier history of Lyme borreliosis and human granulocytic erhlichiosis, was filled in. All those individuals who had participated in a study on LB performed in 1991, and where available blood samples made it possible to compare tick-borne encephalitis immunoglobulin G seroprevalence, were included in the follow-up. Results. A significant increase in immunoglobulin G levels was seen during the follow-up with 24 (12.0%) of 200 blood samples being seropositive in 2002 versus 7 (3,5%) of 200 blood samples in 1991. However, only five participants converted from seronegative level during the 11 y follow-up and one of these participants had been vaccinated against tick-borne encephalitis during the observation period. In only four of all positive sera from 2002 and in no sera from 1991, were neutralizing antibodies against tick-borne encephalitis virus demonstrated. Compared with women, significantly more men were seropositive. In comparison with other age groups the greatest increase was seen in the age group 20 to 29 years. However, most seropostive levels were seen among those >50 years. Conclusion. Although we found seropositive blood samples in this area already in 1991, the existence of tick-borne encephalitis virus at that time is doubtful since no neutralizing antibodies against tick-borne encephalitis virus were demonstrated. During the 11 years follow-up an obvious increase of tick-borne encephalitis immunoglobulin G seropositive levels in humans was seen. Recommending preventing measures, including vaccination against tick-borne encephalitis is of importance for people regularly staying in this endemic area.