No serious side effects with resultant permanent disability were encountered amongst 1457 patients receiving tick-borne encephalitis (TBE) vaccination. However, generalized and local reactions occurred in a high percentage of cases following the first vaccination. Fever, headache and vomiting were frequently encountered together. Over 50% of the children under 10 reacted with pyrexia of varying degree up to 39 degrees C, but the temperature seldom exceeded this mark. These reactions began immediately (usually in the night) after the vaccination and generally disappeared after one to two days (rarely longer). During this time a decrease in work efficiency must be expected in a high percentage of vaccinated patients. On the other hand, complete inability to work was recorded in a small percentage of cases only. After the second vaccination (four weeks later) side effects such as fever appeared significantly less frequently, but local reactions were more common. Hence, limitation of work ability over a short period must be anticipated in a relatively high percentage of cases receiving their second dose of TBE vaccine.