The aetiologies of irritable bowel syndrome and chronic fatigue are unknown. Psychological as well as physical factors have been implicated in both. Fatigue is common in irritable bowel syndrome patients. The purpose of the study was to determine the prevalence of irritable bowel syndrome in chronic fatigue sufferers. A bowel symptom questionnaire was sent to all 4,000 members of a self-help group for fatigue sufferers. Of the 1,797 who responded, 1,129 (63%) fulfilled a diagnosis of irritable bowel syndrome (recurrent abdominal pain and at least three Manning criteria). This greatly exceeds estimates of irritable bowel syndrome prevalence of up to 22% in the general population. Furthermore, irritable bowel syndrome sufferers within this chronic fatigue population reported more Manning criteria (14% had all six Manning criteria) than irritable bowel syndrome sufferers in the general population. This study demonstrates an overlap of symptoms in chronic fatigue and irritable bowel syndrome. In chronic fatigue, irritable bowel symptoms may be one aspect of a more generalised disorder.