The authors report the first Thai family with cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) in which the family members had a classical history of progressive vascular dementia. The proband was a 31-year old Thai male who presented with an acute stroke in the subcortical region. His past history revealed mental disturbance, including poor judgement and regressive behavior as well as mood changes for 1 year. He did not have a history of migraine or any other vascular risk factors except for a strong family history of ischemic stroke and progressive dementia. Magnetic resonance imaging demonstrated multiple small infarctions in the subcortical white matter of the bilateral frontal, parietal and occipital lobes with another small lesion in the pons. Genetic study demonstrated a Notch 3 mutation consisting of the substitution of a nucleotide at position 406 in exon 3 leading to the replacement of an Arginine by Cysteine at position 110 in the 2nd EGF motif, which is compatable with CADASIL.