The first person reported with tick-borne encephalitis (TBE) in Norway fell ill in 1998; the second patient caught the disease in autumn 1999. Both had been to the island of Tromøy in Aust-Agder county. Searches for TBE antibodies were intensified in persons with encephalitis and a seroprevalence study was carried out. Sera from persons with encephalitis were tested for IgM and IgG antibodies to TBE virus with enzyme immunoassay and a neutralization test. Stored sera from persons living on Tromøy in 1997-2000 were studied for IgG antibodies. We report three cases of TBE, two had encephalitis in September-October 2000. The third patient, with antibodies to TBE virus found by retrospective testing, had the disease in August 1997. IgG antibodies to TBE virus were found in 3 out of 126 (2.4%) samples from Tromøy. We report the first case of TBE in Norway. Of the first five Norwegian cases, four had been on Tromøy before they fell ill, three of them as tourists. In previous studies, IgG antibodies to TBE virus were found in 0.3-0.4% of persons from different parts of Agder counties. The seroprevalence studies indicate that Tromøy may have a higher incidence of TBE than the rest of the two southern counties. Our results confirm that TBE occurs in the coastal area of southern Norway.