Dementia is reversible in some cases and these should be diagnosed without over-investigating the many others with irreversible disease. To estimate how often dementia can be reversed, we carried out a quantitative review of studies reported between 1972 and 1994 in which reversible dementia was diagnosed and outcome after treatment was assessed. We found 16 studies comprising 1551 patients. The percentages of reversed dementia varied widely: from 0 to 23% for partial and from 0 to 10% for full reversal. Depression and drug intoxication were the most frequent causes of reversible dementia, followed by metabolic and neurosurgical disorders. The percentage of both partial and full reversal of dementia has fallen in recent years, to less than 1% for both in the four most recent studies. This decrease could be associated with the change from an inpatient to an outpatient setting and the use of stricter diagnostic methods. We conclude that reversible dementia is very rare in an outpatient setting when using strict diagnostic methods. This has important implications for the diagnostic strategy in patients with dementia: major procedures should be performed selectively. In patients with clinical characteristics of Alzheimer's disease, CT of the brain is unlikely to detect a treatable cause of dementia.