Fractional anisotropy from diffusion tensor imaging (DTI-FA) has frequently been used to probe changes in white matter microstructure, but is also heavily affected by axonal fiber dispersion. μFA removes fiber dispersion effects and thereby estimates the microscopic anisotropy. Here, we found lower μFA in normal appearing white matter in multiple sclerosis patients as compared with healthy controls. In addition, μFA correlated significantly with age, disability and cognitive performance. These relations could not be established with DTI-FA. Our results indicate that μFA could be used as a powerful biomarker for diseases related to micro-structural changes in white matter as well as in studies of the healthy brain.