Decline of cerebral blood flow had a significant role in genesis of vascular dementia but recently was suggested to be an etiological factor for development of Alzheimer dementia. The present study was trying to answer this question by estimation of the cerebral blood flow in both vascular and Alzheimer dementia and to compare results with that of healthy elderly. Results showed a lower cerebral blood flow in both types of dementia but, such decline was significant for vascular dementia and insignificant for Alzheimer dementia. Cardiovascular risk factors though were present in both vascular and Alzheimer dementia but were more prevalent in vascular dementia than Alzheimer dementia and this support the idea that correction of cardiovascular risk factors is essential in elderly to prevent or ameliorate cognitive dysfunction which is a major manifestation of dementia.