Epidemiological studies have demonstrated that suffering from depression and bipolar disorder may be risk factors for developing dementia. A mechanism of interactions of several factors, such as vascular disease and glucocorticoid, has been speculated to play a role in the development of dementia. It is suggested that the onset of dementia can be prevented or delayed by preventing the onset and recurrence of depression and bipolar disorder. In the prevent of depression, the management of daily life, such as diet and exercise, is important. Recently, the possibility of preventive effects of antidepressants and lithium on developing dementia has been suggested, and a future intervention study is expected.