Postmortem studies of brains from adults with Down's syndrome (DS) reveal a dramatic age-dependent increase in the incidence of neuropathology associated with Alzheimer's disease (AD). By the age of 40 years, virtually all DS individuals have AD neuropathology. Documentation of cognitive correlates of this phenomenon has been difficult, partly because of the preexisting mental retardation in DS. In the current study, we compared a group of adults with DS, 22 to 51 years old, with a matched control group on various behavioral measures such as savings scores, which are known to be sensitive in detecting early dementia in AD patients. By using the short delayed savings score from the California Verbal Learning Test (a test of verbal memory), a subgroup of DS adults was identified as memory-impaired. This group demonstrated a decline in performance on various other cognitive tests with advancing age, whereas another group identified as having non-memory-impaired DS, and the non-DS controls, showed no evidence of decline with age. These results provide evidence for the presence of early dementia among adults with DS within an age range in which neuropathologic manifestations of AD are predicted to be developing.