We reported a 67-year-old woman who had developed abnormal behavior and dementia from January 21, 2001 and deteriorated to akinetic mutism on February 15. T2-weighted magnetic resonance imaging showed high intensity in bilateral globus pallidus and a rapid spreading of diffuse high intensity in bilateral deep white matter. Later on, we got an important information that she had used a little coal stove three times about three weeks before presenting the initial symptom. She was diagnosed as carbon monoxide intoxication and treated with hyperbaric oxygen from March 1. A remarkable improvement on dementia and motor disability was observed. We conclude that this case is the first reported example of a case of intermittent carbon monoxide intoxication presenting subacute dementia as an initial symptom.