Convergent experimental evidence points to the cerebellum as a key neural structure mediating adaptation to visual and proprioceptive perturbations. In a previous study, we have shown that activity in the anterior cerebellum varies with the rate of learning, with fast learners exhibiting more activity in this region than slow learners. Here, we investigated whether this variability in behavior may partly reflect inter-individual differences in the structural properties of cerebellar white-matter output tracts. For this purpose, we used diffusion-weighted magnetic resonance imaging to estimate fractional anisotropy (FA), and correlated the FA with the rate of adaptation to an optical rotation in 11 subjects. We found that FA in a region consistent with the superior cerebellar peduncle (SCP), containing fibers connecting the cerebellar cortex with motor and premotor cortex, was positively correlated with the rate of adaptation but not with the general level of performance or the initial deviation. The same pattern was observed in a region of the lateral posterior cerebellum. In contrast, FA in the angular gyrus of the posterior parietal cortex correlated positively both with the rate of adaptation and the overall level of performance. Our results show that the rate of learning a visuomotor task is associated with FA of cerebellar pathways. Hum Brain Mapp, 2009. © 2009 Wiley-Liss, Inc.