The frequency of occurrence of tick-borne encephalitis has increased significantly in Europe and Finland during the past twenty years. There has been a suspicion that the Finnish cases have at least over the last few years been more severe than described in the literature. We analyzed the approximately one hundred cases of tick-borne encephalitis diagnosed in Finland over a three-year period. In two thirds of the patients general physical condition deteriorated clearly, the disease caused permanent disability or the patients required supportive measures in the hospital, such as ventilator therapy. Assessment of the course and severity of the disease would seem to be impossible on the basis of individual symptoms, such as duration of fever.