A 69-year-old woman presented with progressive limb weakness lasting 50 years. She denied any congenital disorders or a family history of neuromuscular disease. At ages 29, 46, and 58 years, she underwent hospitalization and evaluations including electromyogram (EMG) and muscle biopsy, but the results were inconclusive. As a result, she received a tentative diagnosis of myopathy of unknown etiology. However, at the age of 69 years, a computed tomography (CT) scan of her skeletal muscles revealed severe involvement of the triceps brachii, iliopsoas, and gastrocnemius muscles, along with preservation of the biceps brachii, gluteus maximus, and tibialis anterior muscles, which was consistent with spinal muscular atrophy (SMA). Finally, genetic testing revealed the deletion of the survival of the motor neuron 1 (SMN1) gene, confirming the diagnosis of SMA type 3. As our case suggests, SMA patients with prolonged disease duration could be underdiagnosed even after EMG and muscle biopsy. A skeletal CT scan could be useful for the diagnosis of SMA patients compared with MRI.