The protein beta-amyloid is said to be central to the disease process of Alzheimer's disease (AD). Several groups have developed transgenic models that overexpress the amyloid precursor protein or beta-amyloid and then develop AD-like neuropathology. Another report suggests that beta-amyloid accumulation in old dogs correlates with cognitive impairment. However, many other researchers argue that beta-amyloid deposition in senile plaques is a secondary event because plaque numbers in man do not correlate well with cognition. We set out to analyse this conumdrum in man. We selected 16 mild to severely demented AD cases on the basis of mini-mental state exam scores (MMSE; n = 16). We also included 4 controls who represented the upper range of cognitive ability. We used a computer-based image analysis of cross-sectional area of the brain occupied by beta-amyloid immunopositive deposition. We used this technique in preference to conventional methods of manual plaque counts and found a strong relation between beta-amyloid load in entorhinal cortex and cognition measured on various scales (r = -0.93 versus the Blessed IMC). Our study suggests that the size of cortical area affected by beta-amyloid deposition is an important factor in the clinical manifestation of dementia, and lends support to the possibility that beta-amyloid is central to the aetiology of AD.