It has been noted the relationship between depression and dementia in elderly. The prevalence of depression in Alzheimer's disease (AD) was reported 20 to 30% in population-based study. Apathy was more frequently observed than depression among demented outpatients in our study. Whereas the apathy has been considered to be a kind of depression traditionally, it has been found that the apathy is distinct from depression recently. In our clinical practice, we frequently realize the close relationship between depression and dementia. It has been reported that risk of AD was 1.71 (odds ratio) in the history of depression more than 25 years ago, and interval between diagnoses of depression and AD was positively related to increased risk of developing AD, suggesting that rather than a prodrome, depression may be a risk factor for AD. Furthermore, Rotterdam Scan Study reported recently that the risk of AD increased to 3.76 in the history of pre-senile depression, while 2.34 in the history of senile depression. On the other hand, it is interesting but still controversial whether depression is associated with the conversion from mild cognitive impairment (MCI) into dementia. The underlying neuropathological condition that causes MCI or dementia might also cause depressive symptoms.