Background/Aims : The aim of this study was to identify differences in anorectal manometry between diarrhea-predominant cases and constipation-predominant cases in patients with irritable bowel syndrome (IBS). Methods: For 62 patients with IBS (29 patients with diarrhea and 33 patients with constipation) and 23 healthy controls, we measured maximum basal and squeeze pressure of anus, threshold of rectoanal inhibitory reflex, rectal compliance and volumes to elicit first sensation, sense of defecation, urgency and maximum toleration. The rectums of subjects were classified into a normal type, a sensitive type, and an insensitive type according to the threshold of sensation and expansion rate of rectum. Results: The volumes to elicit sense of defecation, urgency and maximum tolerable discomfort of patients with diarrhea were significantly lower than those of patients with constipation. Sixteen (55.2%) in the diarrhea-predominant patients with IBS had sensitive rectum but 4 (12.1%) in the constipation-predominant patients with IBS had sensitive rectum. There were no significant differences in other parameters among the two patients groups and controls. Conclusions : There were significant differences in the threshold of rectal sensation and rectal sensitivity between diarrhea-predominant patients with IBS and constipation-predominant patients with IBS. (Kor J Gastroenterol 2000;36:483 - 492)