Pompe disease (type II glycogenosis--GSD II) is a progressive metabolic myopathy caused by lysosomal storage of glycogen due to deficiency of acid alpha-glucosidase. We present the case of a 32-year-old patient with Pompe disease diagnosed 14 years ago in whom enzyme replacement therapy with recombinant human acid alpha-glucosidase (rhGAA) (20 mg/kg i.v. every 2 weeks) has been administered for about 18 months. Despite the fact that therapy was started in the advanced phase of Pompe disease we observed clinical improvement (increased muscle bulk and muscle strength as well as increased range of movements in the distal parts of limbs). In addition, we noticed less effort dyspnoea and use of a respirator during the day shortened to 2-3 hours (previously 5 hours). According to the observation of our patient, we suggest that enzyme replacement therapy causes clinical improvement.