Vascular dementia is the most common cause of dementia in the elderly after Alzheimer's disease. Many forms of vascular dementia have been described: multi-infarct dementia, lacunar dementia, Binswanger's subcortical encephalopathy, cerebral amyloid angiopathy, white matter lesions associated with dementias, single infarct dementia, dementia linked to hypoperfusion and haemorrhagic dementia. The difficulty of diagnosing vascular dementia must not be underestimated and an international consensus is needed for epidemiological studies. The NINCDS-AIREN group has recently published diagnostic criteria. The State of California Alzheimer's Disease Diagnostic and Treatment Centers also proposed some which differ from the NINCDS-AIREN criteria in considering only ischaemic vascular dementia and not other mechanisms such as haemorrhagic or hypoxic lesions. Most studies stress hypertension as the most powerful risk factor for all forms of vascular dementia. The incidence rate ranges from 7 per 1000 person-years in normal volunteers to 16 per 1000 person-years in hypertensive patients. No therapeutic attempt has influenced the course of the disease once the dementing condition is established. The only effective approach is preventive treatment. The objective of the SYST-EUR Vascular Dementia project is to confirm that the treatment of isolated systolic hypertension is able to reduce its incidence.