Objective: To prospectively determine the potential of diffusion MRI (dMRI) of the cervical spinal cord and the corticospinal tracts in brain as surrogate outcome measure for progression of myelopathy in men with adrenoleukodystrophy, as better outcome measures to quantify progression of myelopathy would enable clinical trials with less patients and shorter follow-up. Methods: Clinical assessment of myelopathy included Expanded Disability Status Scale (EDSS), Severity Scoring system for Progressive Myelopathy (SSPROM), timed up-and-go and 6-minute walk test. Applied dMRI metrics included fractional anisotropy, mean diffusivity, axial diffusivity and radial diffusivity. Results: Data was available for 33 controls and 52 patients. First, cross-sectionally, differences between groups (controls vs. patients; controls vs. asymptomatic patients vs. symptomatic patients) were statistically significant for fractional anisotropy, mean diffusivity and radial diffusivity in spinal cord and brain corticospinal tracts (effect size 0.31-0.68). Correlations between dMRI metrics and clinical measures were moderate to strong (correlation coefficient 0.35-0.60). Second, longitudinally (n=36), change on clinical measures was significant after 2-year follow-up for EDSS, SSPROM and timed up-and-go (p≤0.021, effect size ≤0.14). Change on brain fractional anisotropy and radial diffusivity was slightly larger (p≤0.002, effect sizes 0.16-0.28). In addition, a statistically significant change was detectable in asymptomatic patients using brain dMRI and not using the clinical measures. Change on clinical measures did not correlate to change on dMRI metrics. Conclusions: Although effect sizes were small, our prospective data illustrate the potential of dMRI as surrogate outcome measures for progression of myelopathy in men with adrenoleukodystrophy.