Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a hereditary systemic arteriopathy presenting with migraines, mood disorders, focal neurologic deficits, recurrent ischemic attacks and dementia in young adults. The genesis of this disease relates to missense mutation of the Notch3 gene. We report here a newly identified CADASIL patient and discuss unique vascular lesions observed in the kidney. A 64-year-old female was admitted to our hospital for the investigation of proteinuria, hematuria and progressive neurological abnormalities. Her mother and brother died of cerebral infarction at a relatively young age despite a lack of apparent risk factors for arteriosclerosis. Over the past 4 months before admission, she had suffered from frequent transient ischemic attacks despite appropriate antiplatelet therapy. Blood examination revealed mild renal insufficiency and urinalysis revealed moderate protein excretion and dysmorphic hematuria. Magnetic resonance imaging of the brain revealed multiple infarcts and leukoencephalopathy. Histopathological analysis of the kidney revealed focal segmental mesangial proliferation, the loss and degeneration of arterial medial smooth muscle cells and arterial intimal thickening. Immunofluorescence analysis of glomeruli revealed IgA deposition in the mesangial area. Electron microscope analysis revealed electron-dense deposition also in the mesangial area. In addition, granular osmophilic material (GOM) was observed in the extraglomerular mesangial area and around the vascular smooth muscle cells. Genetic analysis of Notch3 revealed an R141C missense mutation and she was diagnosed with CADASIL complicated with IgA nephropathy. In immunohistological analysis, Notch3 stains were positive in vascular smooth muscle cells of the interlobular arteries and both afferent and efferent arterioles, and weak in the glomerular mesangial area. Antihypertensive treatment using angiotensin II receptor blocker and a low protein diet were initiated, and her urinary protein excretion decreased to 0.2 g/day. However, due to the progression of her neurological abnormalities, she became socially withdrawn. In CADASIL, GOM, abnormal accumulation of Notch3 ectodomain, is thought to induce the degeneration and loss of vascular smooth muscle cells and subsequent intimal thickening. Analysis of our cases provided that these morphological abnormalities were also observed in the CADASIL patient kidney.