Numerous similarities in MRI and clinical symptoms exist between Alzheimer's disease (AD), subcortical vascular dementia (sVD) and possible idiopathic normal pressure hydrocephalus (iHPN). The aim of this study is to explore mean apparent coefficient diffusion (ADC) difference between theses diseases in different periventricular and deep white matter areas, as compared to healthy controls. This retrospective study analyzed mean ADC values of 120 patients in normal appearing deep white matter and lenticular nuclei, frontal, caudate nuclei corpus and parietal periventricular and deep white matter areas INPH group showed significantly lower ADC than sVD group in frontal periventricular region (1567.10⁻⁶mm²/s vs 1755.10⁻⁶mm²/s; P=0.0009) and in parietal deep region (1087.10⁻⁶mm²/s vs 1271.10⁻⁶mm²/s; P=0.0052), but showed significantly higher ADC in lenticular nuclei ROI (834.10⁻⁶mm²/s vs 753.10⁻⁶mm²/s; P=0.002). The comparison between iNPH and sVD showed a cut-off value of 1676.10⁻⁶mm²/s (sensitivity 0.70, specificity 0.77) in periventricular frontal area. INPH group, in comparison with NA group, showed significantly higher ADC in all ROIs. The iNPH group also showed significantly higher ADC than AD group in all ROIs. AD group showed significantly lower ADC than sVD group in all regions, except in normal appearing lenticular nuclei and caudate nuclei corpus deep ROI. SVD group showed significantly higher ADC than NA in all ROIs, except in normal appearing lenticular nucleus ROI. Different patterns of ADC values can differentiate between AD, sVD and iNPH, even when other MRI sequences appear morphologically similar.