Amyloid-rich plaques are a feature of the aging human cerebral cortex. We have recently described another feature of aging human cortex, microhaemorrhages, identified by their content of haem, red blood cells, collagen and clotting factors, and their spatial relationship to capillaries. Here we relate microhaemorrhages to amyloid deposits. Observations were made in three groups: patients with no history of dementia, patients with Alzheimer's disease (AD) and patients with Down's syndrome (DS) and dementia. Amyloid deposits and microhaemorrhages were labelled in adjacent sections, amyloid deposits with antibodies to beta-amyloid (betaA), and microhaemorrhages by Prussian blue histochemistry for haem. The densities and sizes of betaA deposits and haem-rich deposits (HRDs), and their relationship to blood vessels, were surveyed in temporal, cingulate and superior frontal cortex. Our results suggest that HRDs and betaA deposits are the same sites of pathology. Their densities in the cortex and white matter of the regions surveyed varied markedly between cases, particularly between demented and non-demented cases, but they always co-varied; where haem deposits were sparse or numerous, so were betaA deposits. Both HRDs and betaA deposits formed adjacent to or encircling small vessels, often at branch points, and a spatial proximity analysis confirmed that both were found close to or colocalising with microvessels. Both HRDs and betaA deposits were associated with blood- or vessel-derived proteins (fibrinogen, von Willebrand factor and collagen VI). Since haem is an established marker of cerebral bleeding, and amyloid is a marker of senile plaques, our results indicate that senile plaques are sites of microhaemorrhages. This colocalisation raises the very testable questions of whether microhaemorrhages are early events in plaque formation and whether therapies which stabilise cerebral microvessels can prevent the onset or slow the progress of dementias associated with plaque formation.