In patients with sleep apnoea syndrome abnormalities of the upper airways may play a major role in the genesis of apnoea. Three cases are presented which illustrate the part played by hypertrophy of the tonsils in a child and by stenosis of the pharynx in an adult and of the larynx in another adult. All three patients were cured by surgical correction of these abnormalities. The physiopathology of alveolar hypoventilation, present in one of the three patients, is discussed in relation to the effects of pure oxygen inhalation on the patient's ventilation.