Japanese encephalitis (JE) vaccine has been used for childhood immunization programmes in Asia since the 1960s. Also, travellers from other parts of the world have been vaccinated before travelling to Asian countries. Some JE vaccines are produced from infected mouse brains and contain small amounts of myelin basic protein. Neurological side effects in larger vaccine trials in Asia have been reported in 1-2.3 per million vaccinees. Statens Serum Institut is the only distributor of JE vaccine in Denmark, delivering 384 000 doses from 1983-96. In 1996, evaluation of initial symptoms and findings in 10 adult travellers from Denmark, who developed moderate-severe neurological symptoms within a few weeks of JE vaccination, was performed as well as follow-up magnetic resonance imaging (MRI) and clinical neurological examination. Three patients initially had symptoms varying from severe encephalitis-like illness to paraesthesia, double vision or parkinsonian gait disturbance. MRI showed severe atrophy of the corpus callosum with altered signal intensity indicating gliosis in one patient, another patient had several hyperintense spots located periventricularly in the white matter, while a third patient had spots with increased signals in the pons, the right substantia nigra and the occipital region. Acute disseminated encephalomyelitis (ADEM) is a possible explanation for these MRI changes, although multiple sclerosis is an alternative diagnosis in one or two of the patients. Another three patients had long-lasting headache, concentration difficulty or intellectual reduction. One man had afebrile convulsions, another gait instability and depression and one parkinsonism. A woman developed myelitis. If these findings are due to JE vaccination the frequency of neurological reactions to the vaccine is considerably higher than previously reported and in the future any minor neurological complaints occurring shortly after vaccination should lead to neurological examination and acute MRI scan should be considered. Copyright 1998 Lippincott Williams & Wilkins