Acid maltase deficiency (AMD) is a metabolic myopathy which may be revealed at adulthood by respiratory muscle weakness, resulting in reduced vital capacity, alveolar hypoventilation and sleep apnoea. We observed two men, 39 and 42 years old respectively, suffering from asthenia and exertional dyspnoea for several months. After a stay in the intensive care unit, as a result of respiratory failure associated with pneumonia, these patients were referred to the respiratory medicine unit on account of persistent hypercapnia during the day and a fall in oxygen saturation at night. The investigations revealed proximal muscle weakness, a reduced vital capacity, alveolar hypoventilation (PaCO2: 67 and 49 mmHg), reduced maximum static inspiratory and expiratory pressures, carbon dioxide hyporesponsiveness and sleep apnoea on overnight polysomnography. Electromyography showed a myopathic pattern. Muscle biopsy confirmed the diagnosis of AMD. Non-invasive ventilation overnight partially corrected the clinical symptoms and the resting hypercapnia in both patients. The adulthood form of AMD is a rare disease that should be considered in a large number of clinical situations, particularly in unexplained respiratory failure. Our observations suggest that non invasive ventilation together with enzyme supplementation (Myozyme®) is effective in correcting alveolar hypoventilation.