We report an 84-year-old woman who suffered from acute fentanyl intoxication followed by delayed encephalopathy. She used fentanyl patch at her ached knee joint and stayed in the covered table with a heat source underneath. Serum concentration of fentanyl suddenly elevated and she developed coma due to acute fentanyl intoxication. She gradually recovered, however, she showed consciousness disturbance again at 15th day and developed neurological symptoms including mutism, pyramidal tract signs, frontal lobe signs and parkinsonism. These symptoms also steadily improved without specific treatment, and became able to talk with colleagues and walk with assistance at 90th day. We considered that delayed encephalopathy due to periodic anoxia caused by fentanyl intoxication occurred in the present patient; however, the neurological sequelae were relatively mild as compared with reported cases of carbon monoxide intoxication. Neuroprotective effect of fentanyl may have contributed to the better prognosis in the present patient.