Background Previous studies have shown that the functional brain activity in the resting state is impaired in Alzheimer's disease (AD) patients.However,most studies focused on the relationship between different brain areas,rather than the amplitude or strength of the regional brain activity.The purpose of this study was to explore the functional brain changes in AD patients by measuring the amplitude of the blood oxygenation level dependent (BOLD) functional MRI (fMRI) signals.Methods Twenty mild AD patients and twenty healthy elderly subjects participated in the fMRI scan.The amplitude of low frequency fluctuation (ALFF) was calculated using REST software.Results Compared with the healthy elderly subjects,the mild AD patients showed decreased ALFF in the right posterior cingulate cortex,right ventral medial prefrontal cortex,and in the bilateral dorsal medial prefrontal cortex.No brain region with increased ALFF was found in the AD group compared with the control group.Conclusions The reduced activity in the posterior cingulate cortex and medial prefrontal cortex observed in the present study suggest that the functional abnormalities of those areas are at an early stage of AD.The ALFF analysis may provide a useful tool in fMRI study of AD.