A 71-year-old woman with a family history of amyotrophic lateral sclerosis (ALS) was investigated for symmetrical, proximal limb and abdominal muscle weakness. Initial examination showed mild proximal muscle weakness in the arms and legs, slightly elevated serum creatine kinase (CK) level, and normal electromyographic (EMG) findings. A myopathy was the presumed diagnosis. Over the next year, weakness became severe and tendon reflexes became unelicitable; no upper motor signs were present. EMG then showed acute and chronic denervation and a muscle biopsy showed target fibers and grouped atrophy. DNA analysis revealed a G72C CuZn-superoxide dismutase (SOD1) mutation. Fasciculations were absent throughout the disease. The patient died 53 months after symptom onset and autopsy revealed loss of lower motor neurons (LMN) and SOD1-positive inclusions. This case expands the phenotypic spectrum of ALS associated with SOD1 mutations to include presenting features that mimic a myopathy.