Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a rare inherited adult onset disease characterised most commonly by cerebral ischaemic events and dementia. It is caused by mutations in the Notch3 gene with most clustering in exons 3 and 4. Whether these mutations have any influence on common sporadic ischaemic stroke or dementia cases has not been investigated, partly hampered by the lack of a readily usable genetic test. An easy to use diagnostic array for CADASIL was designed using various restriction endonucleases for the known mutations in exons 3 and 4 and novel mismatch primers were designed where no such enzymes existed. This array was used to identify the allele frequencies of CADASIL mutations and polymorphisms in selected disease cohorts. Seventy patients with radiologically established sporadic ischaemic stroke and 77 patients from a specialist young dementia clinic were recruited. One hundred and seventeen age and sex matched asymptomatic controls were also identified. The diagnostic array was found to work well. None of the 14 known mutations and three previously identified polymorphisms (C474A, A587G, and C594A) in exons 3 and 4 were present in 140 stroke, 110 dementia, or 234 control chromosomes. Molecular variant C381T occurred with a higher frequency of 0.13, whereas G684A occurred with a lower frequency (0.09) than previously reported, although there were no statistical differences between selected cohorts. In conclusion, a readily usable genetic test for CADASIL has been devised that was used to determine allele frequencies in well characterised cohorts of sporadic stroke and dementia patients. The data suggest that despite the clinical resemblance, CADASIL is not a common masquerading cause of stroke or dementia. The test will enable units locally to rapidly screen patients with suspected CADASIL.