Neuronal intranuclear inclusion disease (NIID) is a neurodegenerative disease characterized by appearance of eosinophilic hyaline intranuclear inclusions. While the main symptoms of adult-onset NIID are dementia or limb weakness, some patients present with encephalitic episodes and transient neurological symptoms. The pathophysiology of these acute, transient symptoms, however, remains unknown. Here, we describe an autopsy case of adult-onset NIID with progressive dementia and transient hemiparesis. The patient was a 70-year-old man without a relevant family history, and initially presented with progressive dementia. He then exhibited transient left hemiparesis at 75 years of age and died of ureteral cancer at 77 years of age. Neuropathological examination revealed the presence of multiple areas of focal spongiosis in the subcortical white matter and patchy myelin pallor of the white matter, as in previous reports. Perivascular areas were preserved. However, even in the damaged white matter, with dense glial fibrillary acidic protein (GFAP)-immunoreactivee astrocytic processes were observed in these areas. GFAP immunohistochemistry revealed decreased density and morphological abnormalities of astrocytes in the affected white matter. These pathological findings might reflect blood–brain barrier impairment and dysregulation of blood flow, which may be related to the pathophysiology of the acute, transient symptoms observed in NIID.