An 80-year-old man (patient 1) was admitted to our hospital with numbness of the right leg and weakness of the lower extremities, predominantly in the right leg, for 1 year previously. Neurological examination revealed moderate weakness without atrophy, and fasciculation in the bilateral lower extremities. No hyperreflexia was noted, and the plantar response was flexor. Neither bulbar palsy nor sensory disturbance was observed. Electromyography (EMG) showed a chronic neurogenic pattern, including giant motor unit potentials and reduced interference in all four limb muscles. MRI images of the cervical and lumbar spines showed severe age-related spondylosis. The clinical and laboratory findings led to a diagnosis of spinal progressive muscular atrophy. Motor paralysis progressed slowly for the following four years, culminating in respiratory failure. A 79-year-man, the younger brother of patient 1 (patient 2), suffered from gait disturbance for 3 years before the admission to our hospital. During the following 3 years, bilateral leg weakness developed, causing difficulty walking. Neurological examination revealed a diffuse mild weakness with atrophy and fasciculation in the bilateral lower extremities; the right deltoid muscle was also mildly weak. Mild hyperreflexia was also noted on the left side, and the plantar response was extensor on the left. EMG showed acute and chronic neurogenic patterns in the four limb muscles. Because the missense mutation c.377 T > C; p.L126S was found on exon 5 of the superoxide dismutase (SOD) 1 gene in this patient, a diagnosis of familial ALS was made. Eight patients reported as familial ALS with the SOD1L126S mutation, including the present cases, all developed an onset of weakness in the lower extremities, but showed few upper motor neuron signs. It is important to consider the possibility of this type of familial ALS in a case of spinal progressive muscular atrophy with late-onset and mild progression, if family history is positive.