Background: Irritable bowel syndrome (IBS) is associated with increased psychological symptoms, early life stressors, and alterations in visceral perception and brain responses to noxious visceral stimuli. The autonomic nervous system (ANS) is a likely mediator for these brain-gut interactions. The few studies directly examining ANS measures have been suggestive of alterations in some IBS patients, but no studies to date have examined the potentially critical variables of sex differences or response to visceral stimulation. Aims: (1) To test differences in ANS function during rest and during a visceral Stressor (rectosigmoid balloon distension) between IBS patients and healthy control subjects. (2) To examin e the role of sex on the autonomic responses of IBS patients. Methods: Baseline autonomic measures were evaluated from 130 Rome I positive IBS patients and 55 h ealthy control subjects. Data were also collected from a subset of 46 IBS patien ts and 16 healthy control subjects during a sigmoid balloon distension study. He art rate variability measures of peak power ratio (PPR) and peak power high freq uency (PPHF) were analysed to assess sympathetic balance and parasympathetic res ponse, respectively. Peripheral sympathetic response was measured by skin conduc tance. Results: IBS patients showed a greater skin conductance response to visce ral distension than controls. IBS patients had higher PPR and lower PPHF across conditions. Male IBS patients had higher skin conductance and PPR than females a nd lower PPHF. Conclusions: IBS patients have altered autonomic responsiveness t o a visceral stressor, with increased sympathetic and decreased parasympathetic activity. These differences are predominantly seen in males. Background: Irritable bowel syndrome (IBS) is associated with increased psychological symptoms, early life stressors, and alterations in visceral perception and brain responses to noxious visceral stimuli. The autonomic nervous system (ANS) is a likely mediator for these brain-gut interactions. The few studies directly examining ANS measures have been suggestive of alterations in some IBS patients, but no studies to date have examined the potentially critical variables of sex differences or response to visceral stimulation. Aims: (1) To test differences in ANS function during rest (2) To examin e the role of sex on the autonomic responses of IBS patients. Methods: Baseline autonomic measures were evaluated from 130 Rome I positive IBS patients and 55 h ealthy control subjects. Data were also collected from a subset of 46 IBS patien ts and 16 healthy control subjects durin ga sigmoid balloon distension study. He art rate variability measures of peak power ratio (PPR) and peak power high freq uency (PPHF) were analysed to assess sympathetic balance and parasympathetic res ponse, respectively. Peripheral sympathetic response was measured by skin conduc tance. IBS patients had a greater skin conductance response to visceral distension than controls. IBS patients had higher PPR and lower PPHF across conditions. Male IBS patients had higher skin conductance and PPR than females a nd lower PPHF. Conclusions: IBS patients have altered autonomic responsiveness toa visceral stressor, with increased sympathetic and decreased parasympathetic activity. These differences are predominantly seen in males.