Dementia is a condition resulting in significant loss of intellectual abilities such as memory capacity, severe enough to interfere with social or occupational functioning. Many diseases can cause the dementia syndrome. Alzheimer disease (AD) and cerebrovascular disease (vascular dementia) are the two most common causes, and some cases of dementia involve both of these disorders. Some potentially reversible conditions, such as hypothyroidism, normal pressure hydrocephalus or vitamin B-12 deficiency, are often known to cause dementia. AD is clinically characterized by progressive cognitive decline associated with impairment in activities of daily living and progresisive behavioural disturbances throughout the disease. Of all cases of dementia, 50-80 % were cases of AD. Patients with symptoms of dementia should be thoroughly evaluated before they become inappropriately labeled AD. This article discusses differential diagnosis for dementia and who does riot have Alzheimer disease.