We prospectively investigated pathological modifications in the corticospinal tract (CST), by diffusion tensor imaging (DTI) in 14 patients with sporadic amyotrophic lateral sclerosis (ALS) and 12 healthy volunteers. We used a validated automated method to accurately measure the in vivo thickness of the cerebral cortex. We found a reduction of precentral cortical ribbon thickness in ALS patients with respect to control subjects. DTI metrics demonstrated disorganization of the CST, as characterized by decreased fractional anisotropy (FA) and increased Apparent Diffusion Coefficient in ALS patients with respect to control subjects. Decreased mean FA values along the CST significantly correlated with clinical measures of pyramidal and bulbar impairment. Quantitative analysis of MR data shows that thinning of the motor cortex coexists with CST damage in ALS patients.