Cu/Zn superoxide dismutase (SOD)-like immunoreactivity (LI) was found within Lewy body-like inclusions (LBIs) in the spinal cords of patients with sporadic amyotrophic lateral sclerosis (ALS) by using an antibody to human Cu/ZnSOD. LBIs were detected in the anterior horn cells in 10 of 20 patients with sporadic ALS. In each of these patients, 7 to 60% of LBIs showed Cu/ZnSOD-LI. No Cu/ZnSOD-LI was detected in intact neurons and glia in the 20 ALS patients, as well as in these cells in 10 normal control individuals. The skein-like inclusions and Bunina bodies, which were found in all of the 20 ALS cases, showed no Cu/ZnSOD-LI. Thus, Cu/ZnSOD appears to play roles in the production and/or degradation process of LBIs.