Using a full length cystatin C cDNA probe and the Alu I restriction enzyme a total of 33 patients with senile dementia, Alzheimer type and 31 Down's syndrome patients have been investigated for the presence of the 630 bp Alu I restriction fragment length polymorphism in the cystatin C gene detected in Icelandic patients with hereditary cystatin C amyloid angiopathy. Results showed that all the patients had normal cystatin C fragment length of 600 bp.