This study evaluated progressive white matter abnormalities in mild traumatic brain injury (mTBI) using diffusion-weighted MRI (DW-MRI). The study analyzed DW-MRI scans taken before and after the football season from 4 high school players with a range of head impacts. It also included scans from non-contact sport athletes and an equestrian with multiple concussions. A bootstrapped z-score analysis detected deviations from normal white matter fractional anisotropy. The analysis found the width of the z-distribution was proportional to the number of head impacts, indicating progressive white matter changes. Players with over 1800 career impacts showed pre-season abnormalities, suggesting chronic damage. The equestrian showed dramatic decreases in white matter fractional anisotropy,