The aim of the study was to elucidate whether clinical progress and neurophysiological changes in patients with Down''s syndrome (DS) are similar to the characteristic findings in patients with dementia of Alzheimer type (DAT). Twenty-two DS patients were evaluated in a longitudinal design with rating scales, psychometric testing, regional cerebral blood flow (rCBF) measurements and EEG. Six deceased patients all showed the Alzheimer type of encephalopathy. Comparisons between the first and final test results indicate that DS patients above 40 years of age displayed a marked decline of scores on rating scales and a progressive cognitive deterioration, involving spatial functions. Furthermore, the older patients showed significant rCBF decreases in parietotemporal areas in both hemispheres together with progressing EEG pathology. It is concluded that elderly patients with DS develop organic dementia with clinical and neuropathological characteristics similar to that in DAT without DS.