A defective blood-brain barrier (BBB) has been postulated to be present in Alzheimer's disease (AD), which would allow circulating beta-amyloid peptide to enter the brain. The authors tested this hypothesis by studying BBB function in 14 individuals with probable AD and 9 elderly control subjects. A computed tomographic method was used to measure blood-to-brain transport (K1), tissue-to-blood efflux (k2), tissue plasma space (Vp), and tissue extracellular space (Ve) of meglumine iothalamate. Repeated-measures analysis of variance indicated no significant differences between the groups for any of the measures. The authors conclude that there is no generalized abnormality of the blood-brain barrier in AD.