This study examined decline in cognitive functions in individuals with Down syndrome (DS) over the age of 40 in comparison to participants of the same age and comparable mental handicap without Down syndrome (NonDS). Both DS (n = 32) and NonDS (n = 31) samples were divided into "younger" (40-49 years) and "older" (50-62) groups. Cognitive processes were examined by tests of general intellectual functioning (Dementia Rating Scale, Peabody Picture Vocabulary Test-Revised, and the Matrix Analogies Test-Expanded form), as well as planning, attention, simultaneous, and successive processing tests taken from Das-Naglieri Cognitive Assessment System. The older individuals with Down syndrome performed more poorly than those in the other three groups. The differences were particularly evident in tasks requiring planning and attention. The possibility of using these tests as indicators of the early signs of Alzheimer's disease is discussed.