Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder affecting motor neurons. We describe a novel L67P mutation located in exon 3 of the Cu/Zn superoxide dismutase gene in a patient with pure lower motor neuron signs. To date, 11 mutations involving exon 3 of SOD1 have been described, including the present one. Our data confirm that variable penetrance and predominant lower motor neuron involvement are common characteristics in patients bearing mutations in exon 3 of the SOD1 gene.