Familial cases of amyotrophic lateral sclerosis are most frequently caused by mutation in the superoxide dismutase-1 (SOD1) gene. We report a heterozygous I113F mutation in a patient with familial ALS characterized by early and predominant bilateral vocal cord paralysis followed by descending spinal cord paresis. Modelling of the mutant SOD1 showed an alteration of the protein secondary structure leading to impaired strength of the dimer interface. This may result in a failure of the protein folding and subsequently generation of toxic intracellular aggregates, suggesting a pathogenic role for the mutation.