Hereditary cerebral amyloid angiopathy is a rare form of the more common, non-genetic disease cerebral amyloid angiopathy (CAA). CAA typically is associated with Alzheimer’s disease and is caused by accumulation of amyloid β-protein (Aβ) in the brain’s blood vessels. Some forms of CAA are caused by rare mutations in Aβ, whereas others can be caused by mutations in the protein cystatin C. The latter case is known as Icelandic hereditary cerebral hemorrhage with amyloidosis because it is caused by a mutation in cystatin C in Iceland. The deposition predisposes these blood vessels to failure, increasing the risk of a hemorrhagic stroke, which eventually often is the cause of death in these patients.