Influenza-associated encephalopathy (IAE) usually occurs in children aged <5 years. Adult cases of IAE are very rare and, thus far, no definite adult autopsy case has been reported. Here, we present the first definite adult autopsy case of IAE. A 76-year-old man presented with sudden coma a day after the onset of fever caused by infection with influenza type A virus. Soon after admission, his condition was complicated by DIC, shock, and multiple organ failure, and he was diagnosed with IAE. Oseltamivir administration and steroid pulse therapy were performed but these proved to be ineffective. The patient died about 24 hours after the onset of encephalopathy. The autopsy revealed massive brain edema and diffuse increase of amoeboid glias without inflammatory cell infiltration. Influenza type A/Hong Kong virus (H3) was isolated from his lungs. Serum IL-6 level was extremely high (35,800 pg/ml; normal, 0.221-4.62 pg/ml). The clinical course, and the laboratory and pathological findings of this adult case resembled those of a typical childhood-onset IAE, suggesting the same pathogenesis. During the influenza season, IAE should be taken into account for differential diagnosis in adult patients with altered mental status and fever.