Background/Aims: Chronic idiopathic constipation has been thought to be related to decreased intake of dietary ttber, and high fiber diet was recommended as an initial therapy for consitipated patients. So we estimated the effects of dietary fiber(psyllium husk) on symptoms and colonic transit time in patients with slow transit constipation. Methods: We measured colonic transit time using radioopaque markers in 74 controls and 68 patients with chronic idiopathic constipation. After 4 week treatment of psy]lium husk(3.25g/pack, t.I.d.), we re-evaluated symptoms and colonic transit time in 37 patients with slow transit constipation. Results: In controls, mean bowel frequency was 5.1/week, total colonic transit time was 16.9 hours, and segmental colonic transits were 4.0 hours for right, 5.1 hours for left, and 7.8 hours for rectosigmoid colon. Among 68 patients with chronic idiopathic constipation, 37 patients had delayed total colonic transit time (mean-t 2SD). In these patients with slow transit constipation, mean bowel frequency was 2.0/week, total colonic transit time was 56.6 hours, and segmental colonic transits were 16.2 hours for right, 22.4 hours for left and 18.0 hours for rectosigmoid colon. After treatment, bowel frequency increased from 2.0/week to 4.3/week, and total colonic transit time decreased from 55.6 hours to 37.7 hours. Right, left, and rectosigmoid segmental colonic transit time also decreased from 16.2 to 9.8 hours, 22.4 to 15.0 hours and 18.0 to 12.5 hours, respectively. We found 3 patients with abdominal bloating and I patient with abdominal pain, but no patients had any serious complications. Conclusions: In patients with slow transit constipation, psyllium husk improved symptoms and colonic transit without serious side effect. (Korean J Gastroenterol 1996; 28:513 - 519)