Wernicke encephalopathy is characterized in its classic form by ophthalmoplegia, ataxia, and mental changes. It may occur frequently in chronic alcoholism. We report a case of Wernicke encephalopathy in a woman who underwent weight reduction diet therapy. A 45-year-old woman arrived at the emergency room presented with mental confusion, ataxic gait and opthalmoplegia. 2 month ago, she was diagnosed with Achilles tendinitis due to obesity at a local hospital. Since then she was prescribed weight reduction diet therapy. She ate only half the amounts of foods compared to her average regular diet during the first two weeks. Later, she ate nothing except drank water. Approximately 7 weeks after weight reduction diet, ataxic gait and diplopia developed. And 8 weeks later, she showed confusion. Neurologic examination revealed alert mentality but, disorientation, bilateral horizontal gaze limitation, gaze-evoked horizontal nystagmus and ataxic gait. Brain magnetic resonance imaging showed bilateral symmetrical enhancement of mammillary bodies. She was diagnosed with Wernicke encephalopathy, and immediate reposition of thiamine was started. One day after admittance, disorientation, opthalmoplegia, ataxic gait were resolved, and on the next day, gaze-evoked nystagmus was resolved dramatically. In this case, iatrogenic nutritional deprivation is thought to be the main cause of thiamine deficiency and subsequent encephalopathy. Our case indicates that a weight reduction diet therapy may be a potential risk factor for Wernicke encephalopathy.