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Mutations in the telomeric copy of the SMN gene (SMN1) are responsible for almost all infantile motor neuron disease (MND). In contrast, the role of the centromeric copy of the SMN gene (SMN2) in MND remains unclear. We searched for deletions of SMN1 and SMN2 in a group of 11 patients with sporadic adult-onset lower motor neuron disease (also referred to as "progressive muscular atrophy") and found an excess of patients carrying homozygous deletions of SMN2 exon 7 (36% versus 5% in the normal population). This result suggests that SMN2 deletions could act as a susceptibility factor for sporadic lower motor neuron disease in adults.