A 60-year-old woman was admitted to our hospital because of difficulty in standing on her toes. Neurological examination showed muscle weakness in both calf muscles. Her serum creatine kinase (CK) level was slightly elevated. MRI revealed hyper-intense signals localized in both the gastrocnemius and soleus muscles. Histological examinations of biopsied muscle specimens showed a marked variation in fiber size, small angular fibers, and hypertrophic and splitting fibers, but no muscle fiber necrosis or regeneration or inflammatory cell infiltration. ATPase stained sections showed small grouped atrophy of type 1 fibers. NADH-TR stained sections showed target/targetoid fibers predominantly in type 1 fibers. Dysferlin immunoreactivity was normal. Follow-up clinical evaluation for one year showed no progression. This patient was diagnosed as having an unknown type of spinal muscular atrophy or benign calf amyotrophy. Sporadic cases characterized by elderly-onset, neurogenic muscular atrophy localized in both calf muscles, and non-progressive course are extremely rare in Japan.