We determined the activity, content and mRNA of Cu/Zn superoxide dismutase (SOD1), and copper ion concentration in a Japanese pedigree of familial amyotrophic lateral sclerosis (FALS) having two basepair deletion in the 126th codon of the SOD1 gene. The activity and concentration of the SOD1 were low in red blood cells from the patients and the unaffected subjects with the SOD1 mutation. The SOD activity stain and Western blot analysis of the brain from one of the patients showed low SOD1 activity and the absence of the mutant SOD1. The mRNA due to the mutant SOD1 gene was, however, confirmed. Availability of the copper ions for oxidative catalytic DNA damage in the brain from the patient was 1.9-fold higher than those in the controls. We propose that the decrease of SOD1 activity and increased copper ions play a role in the neuronal death in this FALS.