Late-onset Pompe disease is an inherited metabolic myopathy with low activity of alpha glucosidase and variable clinical symptoms. In this case report we describe a woman with long standing muscular fatigue and malaise with the diagnosis initially established by pathologic findings in the muscle biopsy. Enzyme replacement therapy is now a treatment option, and a prompt diagnosis is therefore relevant. This disease should be considered in patients with unexplained fatigue and reduced physical capacity, especially in case of concurrent elevated levels of creatine kinase and liver enzymes.