Background: Cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL) is caused by mutations of the Notch3 gene on 19p13. Varying phenotypic expression leads to under recognition and misdiagnosis. Prevalence therefore remains uncertain. We sought to estimate the prevalence of CADASIL in the west of Scotland. Methods: A register for CADASIL was established at a regional neurosciences centre in 2002. All patients with genetically (exons 3, 4, 5, and 6) or histologically confirmed CADASIL residing in two defined administrative health areas were identified. Pedigree members at varying risk of carrying the mutation were also identified and the number of probable Notch3 mutation carriers in the defined population was predicted. Prevalence was calculated for definite CADASIL cases, with and without probable carrier numbers, based upon adult population figures from the 2002 national census. Results: Twenty two individuals from seven pedigrees with confirmed CADASIL and resident in the defined geographical area were identified, yielding a prevalence of 1.98 (95% confidence interval 1.24-3.00) per 100 000 adults. An additional 37 individuals were predicted to be carriers of the Notch3 mutation, yielding a probable mutation prevalence of 4.14 (3.04-5.53) per 100 000 adults. Conclusions: The prevalence of genetically proven CADASIL was 1.98 per 100 000 adults in the defined population. This figure underestimates disease burden.