After a single dose of a commonly used commercial vaccine against early-summer meningo-encephalitis a 22-year-old woman developed neuritis of the nerves to the gravity muscles in the legs and feet. The course of the disease and absent antibodies in serum suggested a cellular immune response. In this case, as generally in postvaccinal neuritis, prognosis proved to be favorable. The estimated incidence of postvaccinal neuritis in general is 1:100,000. The indications for active immunization against early-summer meningo-encephalitis should continue to be narrowly defined.