We identified a novel missense mutation in the Cu/Zn superoxide dismutase gene in a family with amyotrophic lateral sclerosis (ALS). The mutation was a transition of T to C, resulting in a substitution of leucine 126 to serine in exon 5. The family had very unique clinical features of extremely mild severity only in the legs of two male patients with onset of 42 and 52 years old, and their mothers did not develop any symptom even after reaching the age of 80 and carrying the same mutation. The present study suggests that there are other factors that delay or prevent the disease.