The aetiology of the hyperventilation syndrome is reviewed with special emphasis on psychological aspects. Early reports linking overbreathing and the emotions can be found as far back as the 16th century. During the last 50 years research has been carried out into respiration in psychiatric disorders but as far as the hyperventilation syndrome is concerned there have been few psychiatric studies. Though many people believe that hyperventilation occurs as a response to anxiety, it has recently been suggested that it is due to a bad breathing habit. Whichever view should prove to be correct, most people would agree that the distressing symptoms produced by hyperventilation may themselves cause anxiety and exacerbate the hyperventilation, thus setting up a vicious circle. By the time the patient presents, this vicious circle has usually become established.