We report a patient who presents with McArdle's disease at the age of 79. M. E. presents with a 5-year history of leg muscle fatigue on exertion and worsening exercise tolerance. She has difficulty rising from a chair and climbing stairs. She also experiences the “second wind” phenomenon with improved exercise tolerance after a period of rest. Although she describes being a poor runner at school age, functionally she has not been limited until the past few years. There is no relevant family history. Examination is remarkable only for bilateral mild proximal hip flexion weakness. Investigation revealed raised creatine kinase (500 u/l) at rest and serum myoglobin (120 u/l). EMG did not show myopathic changes. Imaging of the lumbar spine revealed marked atrophy of the paraspinal muscles. Muscle biopsy was diagnostic for McArdle's disease with absent enzyme histochemistry for myophosphorylase. McArdle's disease is probably underdiagnosed. It is genetically and clinically heterogeneous. Although the majority of patients present by late teens, rare patients can present in the sixth and seventh decades. It is important to have a high index of suspicion in order to make the diagnosis in the elderly patients with a seemingly progressive myopathy.