The purpose of this study is to investigate whether the diffusional anisotropy of water molecules is disrupted in the pyramidal and extra‐pyramidal regions in patients with amyotrophic lateral sclerosis (ALS). We studied seven patients with probable ALS (four women, mean age ± SD, 57.3 ± 6.2 years old) according to the criteria of the World Federation of Neurology. A control group consisted of 11 age‐ and sex‐matched volunteers (six women, 57.1 ± 4.5) without disorders affecting the central nervous system. Voxel‐based diffusion tensor analysis was made with statistical parametric mapping (SPM99). We also created the normalized corticospinal tractography from the diffusion tensor data. The significant fractional anisotropy (FA) decrease in the ALS group was found in the right frontal subgyral white matter and left frontal precentral white matter. These clusters with significant FA decrease corresponded well to the average group map of the corticospinal tract in a standard reference frame. These results suggested that the combination of voxel‐based diffusion tensor analysis and diffusion tensor tractography might help determine the location of the affected neuronal tissues among ALS patients in a non‐invasive manner. Copyright © 2004 John Wiley & Sons, Ltd.