The responses to inhalation of 35% carbon dioxide (CO2) as a stressor were compared in female irritable bowel syndrome(IBS) patients and healthy controls to assess potential differences in cardiovascular, neuroendocrine and behaviouralresponses to stress. A total of 22 women (12 patients with ROME II defined diarrhoea-predominant IBS and 10 aged-matchedcontrols) were challenged with a single vital capacity breath of 35% CO2 (with 65% oxygen). Beat-to-beat blood pressure andheart rate were recorded prior to, during and after the inhalation. Serum cortisol concentration and behavioural ratings weremeasured pre- and post-inhalation. A typical pattern of responses to CO2 was observed, characterised by a reduction in heartrate and increases in serum cortisol and anxiogenic symptoms; however, these responses did not differ between groups. Bothgroups also demonstrated an increase in systolic blood pressure; however, this response was significantly enhanced in IBSpatients compared to healthy controls (P , 0.05). These findings demonstrate that females with diarrhoea-predominant IBShave an exaggerated pressor response to 35% CO2 stress challenge, suggesting a more stress-responsive sympathetic nervoussystem.