Adults with Down syndrome and early stage Alzheimer's disease showed decline in their ability to selectively attend to stimuli in a multitrial cancellation task. They also showed variability in their performance over the test trials, whereas healthy participants showed stability. These changes in performance were observed approximately 2 years prior to a physician's diagnosis of possible Alzheimer's disease, which was made when they were exhibiting declines in episodic memory suggestive of mild cognitive impairment. Performance on this task varied with the evolution of dementia, showed modestly good sensitivity and specificity, and was relatively easy to administer. Given these qualities this task could be a valuable addition to a neuropsychological battery intended for the assessment of mild cognitive impairment and Alzheimer's disease in adults with Down syndrome.