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Stress is a well-known determinant of cognitive performance in both younger and older adults. However, the moderating effect of pathological aging on this relationship remains insufficiently documented. We hypothesize that mild cognitive impairment (MCI) and Alzheimer's disease (AD) patients will report higher perceived stress than control older adults, when asked to complete an effortful cognitive task. We also hypothesize that the deleterious effect of perceived stress on cognitive performance is more evident in MCI and AD patients.The 131 participants consisted of: 25 functional older adults, 35 nonamnestic-MCI patients, 35 amnestic-MCI patients, and 36 AD patients. They were asked to complete the Victoria Stroop test and report their level of perceived stress at four times: before the test (i.e., baseline) and after each part of the test (three, increasingly effortful levels of cognitive requirement).Overall, perceived stress increased in the most effortful condition-except for AD patients, who reported a decline. A positive main effect of perceived stress on response latency was confirmed. However, this effect became negative in all participants when the model accounted for the difficulty of the task.The results suggest that the ability to perceive the stress levels generated by an effortful cognitive task may be altered in AD. They also suggest that the Stroop task may generate a form of perceived stress favoring engagement, and AD patients may not benefit from its positive effects on performance. They put into questions the usual clinical interpretations.