To evaluate the colonic transit in patients with irri- table bowel syndrome, the authors measured the colonic transit time with radioopaque markers. The segmental colonic transit time with commercially available radioopaque markers (Sitzmarks) was studied in 28 healthy volunteers and 41 patients with irritable bowel syndrome (19 constipation dominant types and 22 diarrhea dominant types). The subjects ingested capsule (20 radioopaque 0-ring) at 9:00 a.m. on 3 successive days while they were fully ambulatory. A plain radiography of the abdomen was obtained on the 4th day. Using the modified Arhan and Wald method, the colonic transit was assessed. The results were as follows: 1) The total colonic transit was 22.4±4.9 h(Mean SE); segmental transits were 8.7±4.2 h for the right colon, 9.9±4.3 h for the left colon, and 3.9±4.8 h for the rectum in the normal control group. Age and sex did not influence transit significantly, 2) The total colonic transit was 50.8±12.0 h in the constipation group and 2. 5±1.7 h in the diarrhea group, and it was significantly different from that of the nor- mal control group. 3) Comparison between the normal contral group and patients with irritable bowel syndrome revealed that the right colonic transit time ratio significantly decreased in the constipation group, while the left colonic transit time ratio increased in the constipation group and significantly decreased in the diarrhea group, and the rectal transit time ratio significantly increased in the diarrhea group, This study suggests that the left colonic segment may be a major lesion site in irritable bowel syndrome.