Idiopathic hyperventilation (IH) is a condition of uncertain aetiology characterized by sustained arterial and alveolar hypocapnia and a plethora of symptoms, the most commonly reported being shortness of breath, and breathlessness. We previously reported that anxiety increases respiratory frequency and minute ventilation with no change in metabolism in normal subjects. In this study, we compared the breathing frequency response to 5% and 7% of CO₂ gas mixtures in normal subjects (n = 13) and in subjects with IH (n = 9), taking into account anxiety and breathlessness in order to determine how breathing patterns may vary with changes in the degree of involvement of higher brain centers because of anxiety and the perception of breathlessness. CO₂ produced a significantly higher value in respiratory frequency (f) in subjects with IH. Subjects with IH also showed lower P_ETCO₂ than normal subjects. During the inhalation of room air, a significant correlation between f and trait anxiety scores was observed in normal subjects (r = 0.49) and IH subjects (r = 0.69). However, the IH group showed no significantly higher trait anxiety in comparison with normal subjects. There was a significant correlation between the level of perceived breathlessness and f during the inhalation of 5% and 7% CO₂, even during the inhalation of room air in IH subjects. This study suggests that an excessive increase in f in subjects with IH may be due to the interaction of two factors, trait anxiety and breathlessness.