Data concerning width and surface of the basement membrane of cortical capillaries in three patients with Alzheimer's disease and five age-matched controls, have been submitted to statistical analysis to ascertain the role of the aging process upon thickening of the basement membrane itself. The analysis showed that there was a significant difference between patients with Alzheimer's disease and controls (P less than 0.01) as to both width and surface of the basement membrane, so suggesting that in these patients aging was not the only factor which thickening of the basement membrane of cortical capillaries was related to.