A family is described in which for three subsequent generations numerous individuals were affected with a progressive neuropsychiatric disease with pyramidal, bulbar and cerebellar symptoms, relapsing course and gradually evolving severe dementia. Post-mortem studies performed on three siblings afflicted with the disease suggest that the remarkably uniform macroscopic picture of the cerebral changes consisting in multiple small cystic infarctions, particularly localized to the central grey and white matter and pons as well as the cortical and central brain atrophy, is caused by an occlusive disease of small intracerebral and leptomeningeal arteries and arterioles. Collected pertinent information concerning the affected family members shows that the illness begins in early adulthood (at 29--38 years of age), affects both sexes and generally lasts for 10--15 years. The only exception so far noticed was a second generation descendant of one of the siblings. This patient died about 5 months after clinical onset of the disease in massive cerebral haemorrhage and showed similar vascular changes as the older members of the family. The disease is considered to be genetically caused and transmitted as a dominant autosomal character. For this apparently new nosological entity the eponym "hereditary multi-infarct dementia" is suggested.