The utility of perfusion-weighted magnetic resonance imaging (PW-MRI) for detecting changes in regional cerebral blood flow (rCBF) in patients with mild cognitive impairment (MCI) and early Alzheimer's disease (AD) was evaluated. Thirteen cognitively normal (CN) elderly subjects, 35 mostly amnestic MCI subjects and 20 subjects with mild probable AD were enrolled. During i.v. injection of gadopentetate dimeglumine, a dynamic T2*-weighted single-shot EPI sequence was conducted using a 1.5-T scanner. Frontobasal (FROB), temporoparietal (TPAR), mesiotemporal (MTMP), anterior and posterior cingular (ACING, PCING), amygdala (AMYG), thalamus and cerebellar brain regions were studied. rCBF was computed from regional cerebral blood volume and arterial input function and normalised to white matter. Images were analysed by manually placed regions of interest using anatomical coregistration. Significant decreases of rCBF were detected in MCI vs. CN in MTMP (-23%), AMYG (-20%) and ACING (-15%) with no further decline in mild AD. In PCING hypoperfusion (-10%) was confined to AD. These hypoperfusional changes are a possible correlate of localised impairment of CNS function. In FROB no perfusion changes were observed between diagnostic groups, but hyperperfusion was observed in mild dementia stages, possibly reflecting functional compensatory mechanisms. These data suggest that PW-MRI detects specific changes in rCBF not only in AD, but also in amnestic MCI, a disorder suggested to largely represent a pre-dementia stage of AD. This method may thus be useful in both research and clinical applications to detect early functional brain changes in the pathogenesis of dementias.