To investigate changes in caliber of vessels in leukoencephalopathy with cerebral amyloid angiopathy (CAA) we performed a histological and morphometric study of cerebral arteries in this disease. We histologically examined changes in cortico-leptomeningeal arteries in five cases of leukoencephalopathy with CAA and compared their morphometrically determined wall-to-lumen ratio [(external diameter-internal diameter) x 0.5/internal diameter] with those of amyloid-negative arteries to estimate stenotic changes. Additionally, we compared wall-to-lumen ratios of medullary arteries in brains with CAA and white matter lesions (WML) (CAA(+)/WML(+), n = 5), subcortical arteriosclerotic encephalopathy without CAA (CAA(-)/WML(+), n = 7), and neither CAA nor white matter lesions (CAA(-)/WML(-), n = 5). Amyloid-positive arteries had thinned walls and dilated lumens. The external diameter and the wall-to-lumen ratio for amyloid-positive arteries was smaller than for amyloid-negative arteries in CAA(+)/WML(+) brains. There was no significant difference in the external diameters among the three groups. The wall-to-lumen ratio for medullary arteries in CAA(-)/WML(+) brains was significantly greater than for CAA(+)/WML(+) and CAA(-)/WML(-), but there was no significant difference between CAA(+)/WML(+) and CAA(-)/WML(-). Amyloid deposition causes degeneration of the tunica media, resulting in thinning of the wall and dilation of the lumen. The tunica media of small arteries is important in regulation of cerebral blood flow with degeneration causing impairment of cerebrovascular autoregulation in response to blood pressure. This impairment may lead to white matter lesions.