Three patients had chronic respiratory disorders: a 42-year-old man with glycogenosis type II was tired, had headaches, poor pulmonary function values and, according to the arterial blood gas values, hypercapnia; a man aged 24 with Duchenne's muscular dystrophy had variable moderate dyspnoea with hypoxia and hypercapnia, and a man aged 64 years with an mitochondrial myopathy complained of dyspnoea and headache but had good blood gas values. The symptoms and abnormalities of the first patient were suppressed by nocturnal ventilatory support through a nasal mask system, the second preferred to refrain from ventilatory support and died a few weeks later and the symptoms of the third patient decreased without ventilatory support. Assessing a ventilatory disorder in patients with a neuromuscular disease is not always simple. Symptoms suggestive of nocturnal hypoventilation may occur in patients without respiratory insufficiency. It is also possible for patients with chronic respiratory insufficiency to be free of symptoms. Determinations of the arterial blood gas values are the most reliable method. Since normal daytime values do not exclude a nocturnal respiratory insufficiency, it is advisable in case of suspicion of nocturnal hypoventilation to measure the arterial blood gas values at night, as well. Nocturnal pulse oximetry does not always adequately reflect the degree of hypoventilation. In view of the positive effects of assisted respiration, adequate diagnostic examinations and early referral to a centre for home mechanical ventilation are advisable.