['Diffusion MRI improves detection of abnormalities in white matter tracts in cerebral palsy (CP). Relationships between diffusion measurements and hand function are largely unexplored. We aimed first to assess microstructure of corticofugal fibers, and second to explore associations between tract injury as assessed by quantitative analysis of diffusion MRI and hand function in children with unilateral CP.', 'In this cross-sectional study, 15 children with unilateral CP (6 boys, median age 12.4 years, min 7.2, max 17) and 24 controls were included (9 boys, median age 12.7 years, min 8.8, max 17.3). Hand function was assessed with the Box and Blocks (B&B) test. Magnetic resonance diffusion data (b value = 1,000 s/mm(2), 45 directions) were collected on a 1.5-T scanner. Fractional anisotropy (FA), mean diffusivity (MD), and tensor eigenvalues were measured bilaterally in the cerebral peduncle (ROI1), the posterior limb of the internal capsule (PLIC, ROI2), and corticofugal fibers connecting these regions.', 'In children with CP, FA in both ROIs and the partial tract corresponding to the affected hand was significantly lower compared to controls. This was caused by an increase in diffusivity perpendicular to the tract. After controlling for age, mean FA contralateral to the affected hand correlated with B&B scores, which was independent of lesion type or number of voxels in the partial tract, cerebral peduncle, or PLIC.'] Does diffusion MRI in corticofugal fibers correlate with hand function in unilateral cerebral palsy?