Pompe disease is a rare metabolic myopathy caused by lysosomal α-glucosidase deficiency. Pompe disease ranges from a rapidly progressive course when symptoms present in infancy to a more slowly progressive rate when symptoms present in childhood or adulthood. This open-label prospective exploratory study investigated the effect of 12 months of recombinant enzyme replacement therapy in 5 adult patients who had already advanced to a very severe stage of Pompe disease. Muscular and respiratory function, quantitative muscle testing and spirometry were assessed. Four patients were tracheostomized. Respiratory parameters did not deteriorate. A moderate improvement in sitting/supine slow vital capacity in 2 patients (from 7% to 11% and 28% to 32% of predicted) and reductions of ventilation support in 2 patients was observed. Three patients, wheelchair bound at baseline, improved sitting and proximal motor function; 2 patients improved in their ability to stand and transfer. The treatment was well tolerated. Alglucosidase alfa may stabilize or even slightly improve muscle strength and respiratory function among patients with severe Pompe disease.