Nineteen cases with verified Hereditary Cystatin C Amyloid Angiopathy are presented. All of the cases had one or more cerebrovascular insults starting at the age of 20-41 years and survived from 10 days to 23 years after the first insult. Progressive dementia was a prominent clinical feature in seventeen cases of whom two presented with dementia. At the last examination the majority had severe dementia and severely abnormal EEG. Anti-cystatin C positive amyloid vascular and perivascular infiltrates were found. The resulting damage to the microvasculature of the brain and secondary hemorrhages and infarctions were considered to be an adequate explanation for the dementia in these cases. Skin biopsies can now probably be used to demonstrate cystatin C positive amyloid deposits conclusively in the tissues of these patients.