Wernicke's encephalopathy is an acute-onset illness characterized by mental confusion, ataxia and ophthalmoplegia due to a thiamine deficiency. Immediate administration of thiamine prevents progression of the disease and reverses brain lesions. We have experienced a case of Wernicke's encephalopathy associated with hyperemesis gravidarum. As Wernicke's encephalopathy is mostly associated with chronic alcoholism, the possibility of Wernicke's encephalopathy may be ignored in young women. We emphasize the need for thiamine supplementation in the patient requiring intravenous alimentation such as hyperemesis gravidarum.