Respiratory impairment is the most important prognostic factor in patients with adult-onset Pompe disease. The effect of long-term enzyme replacement therapy (ERT) on pulmonary function remains unclear.Respiratory parameters (vital capacity (VCmax); forced expiratory volume (FEV1); peak expiratory flow (PEF); and blood gas analysis) were monitored every 6 months during a treatment period of 48-77 months of ERT in six patients with genetically and biochemically confirmed adult-onset Pompe disease. Postural drop of VCmax from sitting to supine position, maximal inspiratory muscle pressure (PImax), mouth occlusion pressure after 100 ms (P0.1), and peak cough flow (PCF) were measured over a period of 12 months. Mean change to baseline were +8.8 % points for ΔVCmax, +6.2 % points for ΔFEV1, and +6.6 % points for ΔPEF after 48 months of ERT. In one patient, a decrease of respiratory parameters with later stabilization was observed under ERT until month 42, but noninvasive ventilation (NIV) had to be initiated due to nocturnal desaturation. In the final 12 months period, progressive diaphragm weakness was detected in 3/6 patients (median change in VC% drop +8 %).ERT seems to stabilize pulmonary function and may delay the requirement for ventilation in patients with late-onset Pompe disease.