Severe complications such as meningoencephalitis are quite rare in the setting of mumps infection. We report here on a 24-year-old soldier with a past history of MMR vaccination who was admitted because of aseptic meningitis. His mental status rapidly deteriorated and swelling of both parotid glands was observed. The MR findings revealed discrete lesions in the brainstem and thalamus. He barely recovered within 2 months. Among the adults with failure to acquire immunity despite MMR vaccination, institutional crowding can lead to cases of potentially fatal CNS complication such as mumps meningoencephalitis.