We used diffusion tensor imaging (DTI) to assess Wallerian degeneration\nof the pyramidal tract within the first 2 weeks after ischemic stroke,\nand correlated the extent of Wallerian degeneration with the motor\ndeficit. Nine patients with middle cerebral artery stroke were examined\n2-16 days after stroke by DTI and T2-weighted MRI. We measured fractional\nanisotropy (FA), averaged diffusivity (Dav), eigenvalues of the diffusion\ntensor and T2-weighted signal in the cerebral peduncle and compared\nthese values between the affected and the unaffected side and between\npatients and six controls. FA was significantly reduced on the affected\nside compared to the unaffected side and compared to the control\ngroup. The largest eigenvalue was reduced, whereas the smallest eigenvalue\nwas elevated on the affected side. There was no significant difference\nin T2-weighted signal and Dav. The decrease of anisotropy correlated\npositively with the motor deficit at the time of DTI study and 90\ndays after stroke. The reduction of anisotropy mirrors the disintegration\nof axonal structures, as it occurs in the early phase of Wallerian\ndegeneration. DTI detects changes of water diffusion related to beginning\npyramidal tract degeneration within the first 2 weeks after stroke\nthat are not yet visible in conventional T2-weighted or orientationally\naveraged diffusion weighted MRI. We demonstrated for the first time\na correlation of early DTI findings of pyramidal tract damage with\nthe motor deficit. DTI can help prognosing recovery of motor function\nafter stroke within the early subacute phase.