During 1959-1987, 126 patients in Aland Islands and main Finland had serologically verified infections with the virus of tick-borne encephalitis (Kumlinge disease). The most useful test for specific virological diagnosis was haemagglutination inhibition-IgM (HI-IgM). Most cases were from Aland Islands and nearby south-western main Finland. Two of the infections were imported. There were three laboratory infections and two transfusion infections. The disease occurred mainly from July to September. Detailed hospital records of 108 of the patients could be obtained. Ninety-five per cent of the patients had headaches, 82% had lymphocytosis of the cerebrospinal fluid, and 81% had high fever. Forty-four per cent had a confirmed biphasic course of disease. Seventeen per cent were severely ill. There were no deaths. The sequels were psychic irritability and fatigue for up to 1 year. There were eight cases of paresis, two of which became permanent.