Cortical disconnection due to white matter damage seems to play an important role in cognitive decline related to neurodegeneration. White matter microstructural alterations can be studied non-invasively with diffusion MRI. In the present study, we used diffusion MRI and tract-based spatial statistics to investigate microstructural alterations associated with mild cognitive impairment, the prodromal stage of Alzheimer’s disease, and Parkinson’s disease. Further, we compared white matter microstructure in patients with Parkinson’s disease with and without mild cognitive impairment. We will present the observed group differences and relate them to other clinical data.