Fenazaquin (4-[[4 (1,1-dimethylethyl) pheynyl]ethoxy]quinazoline) is an insecticide that inhibits NADH ubiquinone oxidoreductase of the mitochondria, which is also known as complex I. An 83 year old female was brought to our emergency department (ED) having been found collapsed and unconscious at home by her family. She had ingested up to 100 ml from a bottle of 20% fenazaquin solution. In the ED, she showed severe persistent lactic acidosis despite a seemingly stable hemodynamic condition. Despite intensive supportive management, including positive pressure ventilation, packed red cell transfusion, hemodialysis, and intravenous N-acetylcysteine administration, the lactic acidosis did not respond. To our knowledge, this is the first report of fenazaquin poisoning in humans. No antidote for fenazaquin is known. In this case report, we discuss clinical characteristics and possible pathophysiologic mechanism of fenazaquin poisoning with a literature review.