Differential diagnosis of dementing diseases is very important to rule in the so-called treatable dementia. The new DSM-IV criteria for dementia include memory disturbances and one or more of aphasia, apraxia, or frontal lobe dysfunctions as essentials. Alzheimer disease requires, in addition, slowly progressive course and ruling out other brain or systemic diseases. Vascular dementia requires focal neurological or neuroimaging signs. Other diseases which cause dementia include chronic subdural hematoma, infection and brain tumor. CT or MRI can readily diagnose them if suspected and they may be treated. Systemic diseases associated with treatable dementia include electrolyte disturbances, hypothyroidism, vitamin deficiency, alcohol or drug intoxication, syphilis and HIV infection. Prevention of dementia seems to be the future problem as we could prevent cerebrovascular diseases by treating hypertension.