Acute disseminated encephalomyelitis (ADEM), which is a disease that causes multifocal inflammatory demyelination of the central nervous system, occurs predominantly in children and young adults. We report an autopsy case of an elderly man with brainstem ADEM that progressed over a period of about 3 months. An 82-year-old man developed disturbance of consciousness, dysphagia, and ataxic gait over a period of about 3 months. He was admitted to another hospital for aspiration pneumonia and recovered but was transferred to our hospital due to prolonged disturbance of consciousness. The patient was able to follow simple commands but had a tendency to somnolence. In addition to meningeal stimulation signs, the patient had left-dominant upper and lower limb ataxia and right-dominant limb spasticity. Brain FLAIR/T2-weighted imaging showed high-intensity lesions from the brainstem to the middle cerebellar peduncle bilaterally, medulla oblongata and upper cervical spinal cord, and T1-weighted imaging revealed contrast-enhanced lesions in the left middle cerebellar peduncle and cervical spinal cord. Although spinal fluid examination revealed elevated proteins, other laboratory tests indicated no evidence of infection, vasculitis, collagen diseases or tumors, and anti-ganglioside, anti-AQP4 and anti-MOG antibodies were negative. After admission, the patient again developed aspiration pneumonia, which progressed to acute respiratory distress syndrome, and he died on the 15th day of hospitalization. Autopsy findings indicated acute and subacute demyelination mainly in the brainstem and cerebellum, and perivascular lymphocyte and macrophage infiltration in the areas of demyelination. A postmortem diagnosis of ADEM was made based on the generally monophasic course of the disease and the absence of regenerating myelinated sheaths. There are very few reports of elderly patients with brainstem ADEM. ADEM should be considered as a differential diagnosis in patients with brainstem encephalitis.