BACKGROUND/AIMS: The aim of this study was to find out the difference between a normosensitive and a sensitive rectum to identify the factors related to rectal hypersensitivity in patients with diarrhea-predominant irritable bowel syndrome (IBS). METHODS: Thirty-three patients with diarrhea-predominant IBS were included. Their rectal compliance and balloon volumes to elicit first sensation, sense of defecation, urgency and maximum toleration were measured by rapid phasic distension. The subjects were classified into normal, sensitive and hyposensitive groups according to the results of rectal compliance and the threshold of sense. Parasympathetic function and psychological variables were also assessed. RESULTS: There were no significant differences in the ratio of sex and the frequency of abnormal psychological profiles between the normosensitive group and the sensitive group. There were significant differences in age (p=0.015) and parasympathetic function (p=0.038) between the normosensitive group and the sensitive group. CONCLUSION: Rectal hypersensitivity in patients with diarrhea-predominant IBS was significantly related to age and parasympathetic function.