Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a hereditary stroke disorder. CADASIL is caused by missense point mutations or small deletions of the Notch3 gene, which encodes a large single-pass transmembrane receptor. Notch3 is essential for the normal maturation of blood vessels in both fetal and adult brains in mammals. Typical clinical manifestations are recurrent subcortical ischemic stroke, subcortical dementia, and migraine with aura. The age at the onset of stroke is approximately 40-50 years. Brain MRI shows a characteristic appearance with abnormalities, such as white matter hyperintensities in the anterior temporal lobes and subcortical lacunar lesions. Morphologically, CADASIL is characterized by the degeneration of vascular smooth muscle cells and accumulations of granular osmiophilic material(GOM) and the extracellular portion of Notch3. The progressive degeneration of smooth muscle cells in small blood vessels could be caused by an abnormal accumulation of the Notch3 ectodomain. Diagnostic criteria for CADASIL are the presence of mutations in the Notch3 gene and/or deposits of GOM or the Notch3 ectodomain in blood vessels. The most definitive diagnostic test is genetic testing for the mutated Notch3 gene. It has been shown that almost 70% of mutations can be found within exons 3-4 of the 33 exons making up the gene. Skin biopsies are usually used for the diagnosis, since pathomorphological changes in the small vessels are observed not only in the brain, but also in the skin. Recently, Notch3 immunostaining of skin biopsy specimens has been introduced as a simplified supportive test for the diagnosis of CADASIL.