Severe cortex lesions in the hippocampal, parahippocampal and medial temporal lobe (MTL) of Alzheimer's disease subjects have been observed by functional magnetic resonance imaging (fMRI) during memory task performance. To date, fMRI technology has not been used to investigate the frontal lobe function of Alzheimer's subjects. This study determines if fMRI can be used to assess altered prefrontal cortex activity during Stroop task performance in subjects wih mild cognitive impairment (MCI) and Alzheimer's disease (AD). Functional magnetic resonance imaging (fMRI) was performed on 9 healthy elderly controls, 9 subjects with mild cognitive impairment and 10 patients with Alzheimer's disease, to examine the prefrontal changes in fMRI activation in relation to the Stroop color-word paradigm. In comparison with controls, MCI subjects showed distinctly increased cortex activity including: the dorsal anterior cingulate, bilateral middle and inferior frontal gyri, bilateral inferior parietal lobule, and the bilateral insular. In contrast, AD subjects exhibited decreased fMRI responses in the regions of the prefrontal cortex listed above. These results imply two different neurophysiological characteristics of MCI and AD. In MCI, a compensatory activity of the prefrontal cortex is observed, whereas in AD a dysfunction of the prefrontal cortex is indicated.