Two hypotheses were tested: (a) lowered tolerance for balloon distention of the rectosigmoid in patients with irritable bowel syndrome is caused by a psychological tendency to exaggerate the painfulness of any aversive stimulus, and (b) contractions elicited by balloon distention are responsible for pain reports. Tolerance for stepwise distention of a balloon in the rectosigmoid was compared with tolerance for holding one hand in ice water in 16 irritable bowel patients, 10 patients with functional bowel disorder who did not satisfy restrictive criteria for irritable bowel, 25 lactose malabsorbers, and 18 asymptomatic controls. Contractile activity was measured 5 cm above and 5 cm below the distending balloon. Psychometric tests were used to assess neuroticism, anxiety, and depression, and a standardized psychiatric interview was administered. Patients with irritable bowel syndrome had significantly lower tolerance for balloon distention but not ice water, and balloon tolerance was not correlated with neuroticism or other psychological traits measured. Rectosigmoid and rectal motility were also not related to tolerance for balloon distention. Both hypotheses were rejected. A peripheral mechanism such as altered receptor sensitivity may be the cause of distention pain in irritable bowel syndrome.