Delegates attending the Fourth International Symposium on Respiratory Psychophysiology indicated on a questionnaire those features of personality and mental disorders, symptoms, signs and measurements which they felt to be essential, closely associated, occasionally associated or irrelevant in the diagnosis of the hyperventilation syndrome (HVS). They also supplied their own definition of the HVS. Anxiety, symptoms reproduced in whole or in part by voluntary hyperventilation, breathlessness, hyperventilation and low CO2 were the features from the questionnaire most commonly considered to be essential in the diagnosis of the HVS. Delegates used the following phrases most frequently in their own definitions: "a variety of somatic symptoms", "hyperventilation", "associated low PCO2", "no organic disease" and "physiologically inappropriate". A consensus definition might read "the hyperventilation syndrome is a syndrome characterized by a variety of somatic symptoms induced by physiologically inappropriate hyperventilation and usually reproduced in whole or in part by voluntary hyperventilation". But for practical use, the nature of the symptoms and any other relevant features need to be clearly identified if this diagnosis is to be entertained in the course of a clinical consultation. This definition still lacks the qualities which are essential for an operational definition.