Korsakoff's disease is a brain disorder which is named after Sergei Korsakoff (1854-1900), a Russian neurologist who first described the disease. It arises from a dietary deficiency of thiamin (Vitamin B-1), which the body uses to convert carbohydrates into energy. Over time, thiamin deficiency can cause damage to several brain areas critical for memory: including the thalamus, mammillary bodies and basal forebrain. In rare cases, this damage culminates in Korsakoff's disease.

A common cause of Korsakoff's disease is alcoholism. Alcohol interferes with the body's ability to metabolize thiamin; additionally, alcoholic individuals tend to have poor diets, further reducing intake of thiamin.

Individuals with Korsakoff's disease tend to show general intelligence which is just as good as chronic alcoholics without the disease - which is to say, slightly impaired but not abnormal. Attention may also survive quite well. However, Korsakoff's disease can include both anterograde amnesia (loss of ability to form new memories) and retrograde amnesia (disruption of pre-existing memories). One interesting aspect of Korsakoff's disease is that patients become unable to use time relationships: thus, asked about his autobiography, a Korsakoff's patient may report serving in the army before entering high school. Both recollections may be correct, but their ordering is wrong, and the patient may be unable to perceive the error.

Although Korsakoff's patients may show severe memory deficits, they tend also to show other impairments associated with chronic alcoholism, including disorientation for time and place, apathy, and emotional blandness broken by sudden, momentary irritability, anger or pleasure. Thus, if a Korsakoff's patient performs poorly on a particular neuropsychological test, it is difficult to assess whether this poor performance reflects a true deficit or merely a lack of attention during the test-taking. For this reason, many researchers studying anterograde amnesia prefer not to lump Korsakoff's patients together with other patients who sustained more localized memory impairments as a result of stroke or other injury.

Some improvement can be seen in Korsakoff's patients given thiamin replacement early in the course of the disease, but most patients who develop serious symptoms (e.g. amnesia) do not regain enough capacity to allow independent living. While the non-memory symptoms of the disease often improve following thiamin treatment, the amnesia typically does not.