Spinal cords of sporadic cases with amyotrophic lateral sclerosis (ALS) and normal controls were immunohistochemically examined using antibodies for Cu/Zn superoxide dismutase (SOD) and nitrotyrosine (NT). Immunoreactivity for Cu/Zn SOD of the motor neurons was not different between the ALS and controls. In contrast, immunoreactivity for NT was densely detected in motor neurons of ALS but was not or was only minimally detected in those of controls. The staining was also found in the axons of motor neurons of ALS, but was not found in the controls. These results suggest that nitration of protein-tyrosine residue is upregulated in motor neurons of the spinal cord of ALS.