Between 1991 and 2000 about 1500 patients fell ill in Germany (Baden-Wuerttemberg, Bavaria, Hesse) after infection with the tick-borne encephalitis (TBE) virus. Detailed clinical and epidemiological data of TBE were available from 850 patients. A biphasic course of the disease occurred in 75% of patients. TBE presented as meningitis in 400 patients (47%), as meningoencephalitis in 356 (42%) and as meningoencephalomyelitis in 93 (11%). Nine of the patients (1%) died from TBE. Five hundred and ninety three patients (70%) had noticed a tick bite and the first symptoms occurred, on average, seven days later. The most frequent neurological symptoms were an impairment of consciousness (31%), ataxia (18%) and paresis of the extremities (15%) and cranial nerves (11%). Laboratory investigations revealed leukocytosis in the peripheral blood in 74% of the patients, elevation of the sedimentation rate in 91%, increased C-reactive protein in 82%, pleocytosis in the CSF of all patients tested, damage of the blood-CSF-barrier in 79%, abnormalities in EEG in 77% and abnormalities in MRI in 18%. In general, adolescents up to 14 years of age had a more favourable course of the disease than adults. In view of the severity of the illness and the high frequency of sequelae, active immunisation against TBE is recommended for all subjects living in and travelling to areas of risk. Regarding the higher risk of severe manifestations of TBE in the elderly patients and the higher risk of failure of immunisation in this population, vaccination against TBE is more important in elderly subjects than in very young persons. Prevention of TBE by post exposure prophylaxis with hyperimmunoglobulins is less effective and should therefore not be performed.