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Background and Purpose: The aim of this study was to assess the repeatability of neurite orientation dispersion and density imaging in healthy controls (HCs) and traumatic brain injury (TBI). Methods: Seventeen HCs and 48 TBI patients were scanned twice over 18 weeks with diffusion imaging. Orientation dispersion (ODI), neurite density (NDI), and the fraction of isotropic diffusion (F‐ISO) were quantified in regions of interest (ROIs) from a gray matter, subcortical, and white matter atlas and compared using the coefficient of variation for repeated measures (CVrep), which quantifies the expected percent change on repeated measurement. We used a modified signed likelihood ratio test (M‐SLRT) to compare the CVrep between groups in each ROI while correcting for multiple comparisons. Results: NDI exhibited excellent repeatability in both groups; the only group difference was found in the fusiform gyrus, where HCs exhibited better repeatability (M‐SLRT = 9.463, p =.0021). ODI also had excellent repeatability in both groups, although repeatability was significantly better in HCs in 16 cortical ROIs (p <.0022) and in the bilateral white matter and bilateral cortex (p <.0027). F‐ISO exhibited relatively poor repeatability in both groups, with few group differences. Conclusion: Overall, the repeatability of the NDI, ODI, and F‐ISO metrics over an 18‐week period is acceptable for assessing the effects of behavioral or pharmacological interventions, though caution is advised when assessing F‐ISO changes over time. [ABSTRACT FROM AUTHOR]