Reversible dementia is rare and accounts for approximately 1.5% of all dementias. Systematic ancillary investigations aimed at detecting an infectious disease, an endocrine aetiology or a vitamin deficiency are rarely contributive, but remain relevant since reversible dementia may, very rarely, mimic Alzheimer-type dementia. Aetiological investigations are much better selected and contributory when they rely on a precise analysis of the clinical picture (past medical history, age of the patient, cognitive, psychiatric and behavioural symptoms, type of onset, and associated signs) and of cerebral imaging. Discovering a reversible cause of dementia does not always mean that the patient will completely recover; thus it is more appropriate to use the term "potentially reversible dementia". Finally, when the patient does not recover from dementia, systematic ancillary investigations can identify and treat concomitant reversible conditions, which in nearly 25% of dementia cases contribute to worsening the condition.