Due to the increase of the life expectancy the prevalence of the patients with dementia in the services of Internal Medicine is very high (30% in the patients with more of 80 years old). The incidence of the potentially reversible dementias is very different in the publications consulted (10-30%). The objective of these study is to know the real incidence of these dementias and which is the best diagnostic protocol to optimize the resources. The conclusions obtained in the bibliographical review it is that the effect of this type of dementias is minor that the mentioned (1.5-3%).