Sixty‐seven subjects with karyotypically verified Down's syndrome (DS+) and 48 matched controls without DS (DS− were examined on adaptive, intellectual, memory, orientation and motor functions during a period of 4½ years. The primary analysis compared changes in cognitive and adaptive functions in the two groups. Analysis of variance revealed that in adults with DS, over the age of 50 years, functional loss was statistically significant. Intelligence, memory and orientation functions deteriorated statistically significantly in 2½ years. The decrease in adaptive and motor functions took 4½ years to become statistically significant. No ageing effects were found in control subjects.Further investigation focused on the DS+ population over 50 years old (n = 28). The effects of dementia and sensory deficits on adaptive and cognitive decline were examined. Dementia was found to have a significant deteriorating influence on the majority of the adaptive and cognitive functions under study.Compared to the deteriorating influence of dementia, the impact of hearing and visual deficiencies on functional loss was limited. Only visual impairment was related to a lesser performance of personal daily living skills. Hearing handicaps were not significantly related to functional decline.