Korsakoff's syndrome is a disorder that primarily affects the memory system in the brain. It usually results from a deficiency of thiamine (vitamin B1), which may be caused by alcohol abuse, dietary deficiencies, prolonged vomiting, eating disorders, or the effects of chemotherapy.

Most patients first develop a delirium, marked by confabulation, gait disorder and problems with memory and disorientation (that is usually called a Wernicke's encephalopathy). When this delirium clears, a profound disorder of memory remains. Patients have great difficulty learning and retaining new information as well as problems recalling memories from the recent past.

The incidence of Korsakoff's syndrome has dropped significantly since the fortification of several foods, such as cereals and bread, with thiamine.