BACKGROUND AND PURPOSE: Many studies have reported abnormal changes in relapsing-remitting multiple sclerosis (RRMS) by histogram and region-of-interest-based methods by using diffusion tenor imaging. However, there are few studies on specific white matter fiber tracts of BBMS. Our study sought to use diffusion tensor tractography-based group mapping to investigate the presence of abnormal diffusion in the normal-appearing pyramidal tract (PYT) of BBMS and its possible mechanism. METHODS: A PYT probability map was first constructed from data on 20 healthy patients based on the deterministic-based tractography method. The PYT probability map was then applied to 29 RRMS patients to calculate diffusion indices of the PYT. In this study, 4 quantitative indices-fractional anisotropy (FA), directionally averaged diffusion coefficient (D-av), axial diffusion coefficient (lambda(1)), and radial diffusion coefficient (lambda(23))-were used to characterize the abnormal diffusion. RESULTS: Compared with healthy controls, RRMS patients had a significantly higher D-av and lambda(23) but a lower FA and a trend toward a lower lambda(1) in the normal-appearing PYT. In RRMS patients, PYT lesions had a significantly higher lambda(23) and a lower FA, but there were no differences for D-av and lambda(1) when compared with the normal-appearing PYT. Moreover, the diffusion indices derived from the normal-appearing PYT were significantly correlated with PYT lesion volumes by using the Spearman correlation analysis. CONCLUSION: Our findings confirm the presence of abnormal diffusion in the normal-appearing PYT of RRMS patients and suggest that wallerian degeneration might be its mechanism.