Alzheimer's disease produces a generalized slowing of cognitive processing increasing with the progression of dementia. However little is known about this phenomenon in the pre-demented stages. Our purpose was to investigate cognitive slowing in pre-demented subjects and their ability to develop target detection skills while performing a cancellation task. The results on 16 pre-demented subjects and 32 matched controls show that cognitive slowing is already present 3 years before subjects fulfill dementia criteria, at a stage where errors are still not different between pre-demented and control subjects. Additionally, the two groups significantly improved their time performances all along the test suggesting that both groups including the pre-demented group were able to progressively acquire a target detection skill.