This study evaluated the damage to the extensive range of white matter tracts in patients with Alzheimer's disease (AD) and mild cognitive impairment (MCI). Thirty-four patients with AD (mean age 71.5 yrs, MMSE 17.6), 23 patients with MCI (mean age 66 yrs, MMSE 27.4) and 15 normal controls (mean age 69 yrs, MMSE 29.8) were enrolled. Diffusion tensor imaging (DTI) was performed in 25 directions on 1.5 T MR scanner. Fractional anisotropy (FA) values were obtained with a small ROI method in several association tracts including posterior cingulum fibers, in commissural tracts (genu and splenium of corpus callosum) and projection tracts (middle cerebellar peduncles and posterior limbs of internal capsules). In MCI significant reductions of FA were found in the inferior longitudinal fascicles, left superior longitudinal fascicle and posterior cingulum fibers compared to normal controls. In AD significantly decreased FA values were detected in the same fascicles as in MCI and additionally in inferior fronto-occipital tracts and commissural tracts. In both AD and MCI the most severe changes were found within posterior cingulum fibers. No abnormalities were detected in projection tracts in both groups. Accuracy of DTI in detecting AD and MCI reached 0.95 and 0.79, respectively. FA measurements strongly correlated with neuropsychological tests. DTI is capable of depicting microstructural changes within white matter fiber tracts in dementia and may aid the differential diagnosis of AD and MCI.