The relationship between degree of cerebral amyloid angiopathy (CAA) and the amount of smooth muscle cells (SMCs) and deposition of collagen IV fibres (COL IV) was investigated in the frontal and occipital cortex of 70 patients with autopsy confirmed Alzheimer's disease (AD). The extent of CAA was significantly greater in occipital than in frontal cortex, although SMC loss was greater in frontal than in occipital cortex. COL IV staining was significantly higher in occipital than in frontal cortex. The degree of SMC loss correlated with CAA, as Aβ40 but not as Aβ42 or total Aβ, in frontal cortex, but not in occipital cortex. Leptomeningeal arteries within occipital cortex showed significantly greater external diameter, greater wall thickness and greater luminal area than those in frontal cortex. The degree of CAA correlated with thickness of blood vessel wall and external diameter in frontal cortex, whereas extent of SMC loss correlated with thickness of blood vessel wall in occipital cortex. There were significant negative correlations between duration of disease and thickness of vessel wall, external diameter and luminal area. In patients with disease durations exceeding 10 years, external vessel diameter and thickness of the vessel wall were both halved compared with patients with durations less than 5 years; luminal area was reduced by about 75%. Blood vessels in AD undergo degenerative changes involving deposition of Aβ and COL IV with loss of SMC. SMC loss may relate to increasing Aβ deposition in early stages of disease, but this relationship may be lost with disease progression.