Up to 60% of patients with IBS have lowered perception thresholds in the rectum to balloon distension. The current study sought to test the hypothesis that IBS patients with normal perception thresholds in the rectum show hypersensitivity of afferent pathways in the sigmoid colon. Eleven healthy normal subjects and eight IBS patients with normal rectal perception thresholds underwent a balloon distension protocol in the sigmoid and rectum. Discomfort thresholds, receptive relaxation, compliance, and referral patterns were measured. Although IBS patients had significantly lower discomfort thresholds in the sigmoid when measured as volume, pressure, and wall tension, thresholds were similar to normals. Receptive relaxation and dynamic compliance were significantly decreased in IBS patients in the sigmoid. Referral patterns were similar during sigmoid distention in IBS patients in comparison to normals. Despite normal perception thresholds in rectum and sigmoid, IBS patients show evidence for alterations in rectosigmoid afferent mechanisms. In the sigmoid, this is seen in the form of reduced reflex relaxation and compliance and in the rectum in the form of altered viscerosomatic referral.