We examined 11 subjects with inherited amyotrophic lateral sclerosis (familial amyotrophic lateral sclerosis, FALS) associated with the most common copper/zinc superoxide dismutase 1 (SOD1) mutation, an alanine for valine substitution in codon 4 (A4V). Autopsies were performed on 5 subjects. The clinical and pathological findings are described and compared with those of 9 sporadic ALS (SALS) subjects. There was no clinical evidence of upper motor neuron (UMN) involvement in 10 FALS A4V subjects. All subjects had lower motor neuron (LMN) signs and electrophysiological evidence of denervation in at least three limbs. All SALS subjects had signs of both UMN and LMN involvement. Pathological studies found severe abnormalities of LMNs in all FALS and SALS subjects. UMN involvement was either absent or mild in the A4V SOD1 FALS subjects and severe in the SALS subjects. Pathological abnormalities in systems other than the motor neurons were more frequent in the FALS A4V subjects. This information suggests that current diagnostic criteria for ALS, requiring dinical evidence for both upper and lower motor neuron involvement, should be modified; ie, the diagnosis should be deemed established when there is evidence of denervation in three or more limbs and a mutation in the gene for SOD1, even without dinical signs of UMN involvement.