Cerebral small vessel disease (CSVD) is frequently observed among the elderly and is known to cause dementia and gait disturbance associated with white matter lesions, lacunar infarcts, and cerebral hemorrhage. Molecular mechanistic studies promise to provide new insights into the pathogenesis of hereditary CSVD. Cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL) is one of the hereditary CSVDs caused by a mutation in the high-temperature requirement serine peptidase A1 (HTRA1) gene. The loss of HTRA1 protease activity increases signaling via transforming growth factor (TGF)β, thereby resulting in CARASIL. Although the CARASIL has been characterized by juvenile onset alopecia and spondylosis deformans, these features are not always observed in individuals with an HTRA1 mutation. Moreover, some HTRA1 mutations cause CSVD in heterozygous states. Therefore, the clinical features of CSVD resulting from an HTRA1 mutation extend to patients with CSVD alone or to those with dominantly inherited CSVD.