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To test the hypothesis that there is greater asymmetry in diffusion properties between right and left pyramidal tracts in patients with congenital hemiparesis than in patients with normal motor function. Four congenitally hemiparetic patients and four age-matched controls underwent magnetic resonance diffusion tensor imaging (DTI)-based three-dimensional tractography of the pyramidal tracts. Relative anisotropy, individual eigenvalues, and directionally averaged apparent diffusion coefficient were measured and degree of asymmetry was calculated. Compared with age-matched controls, congenitally hemiparetic patients had greater asymmetry in all measured diffusion properties. The asymmetry was characterized primarily by lower anisotropy, lower parallel diffusion, higher transverse diffusion, and slightly higher mean diffusivity in the pyramidal tract contralateral to the hemiparesis (i.e., affected pyramidal tract) compared with the unaffected pyramidal tract. There appears to be greater diffusion asymmetry between the pyramidal tracts in congenitally hemiparetic patients compared to controls. These differences suggest that there are alterations in the microstructure of the pyramidal tract that controls the motor function of the hemiparetic side. Our results suggest that DTI-based three-dimensional tractography is potentially useful in the assessment of motor dysfunction in infants and children with congenital hemiparesis. J. Magn. Reson. Imaging 2003;18:641–648. © 2003 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]