OBJECTIVE To evaluate the effect of cisapride on the total and segmental colonic transit times (CTT's) and on the clinical symptoms in patients with chronic idiopathic constipation, and to elucidate whether the correction of the CTT parallels the symptomatic improvement. METHODS An open prospective trial of cisapride, 10 mg t.i.d. orally for 8 weeks, was done in 25 adult patients (M: F6: 19) with chronic idiopathic constipation of 1 year or longer duration. CTT was measured at baseline and after 8 weeks of cisapride therapy. Frequency, consistency and difficulty in passage of bowel movements were evaluated at baseline and after 2, 4 and 8 weeks of cisapride therapy. RESULTS 19 cases continued to participate until the end of 4 weeks of therapy and 15 cases completed the entire 8 weeks' trial. No serious side effect was experienced during the study period. Total and right segmental CTT's shortened with 8 weeks of cisapride treatment. Defecation frequency increased and difficulty in stool passage improved with 2 to 8 weeks of treatment. Stool consistency had a tendency toward normal after 8 weeks of treatment (p = 0.06). The global response was excellent in 7 cases (46.7%), good in 5 (33.3%) and poor in 3 (20.0%). If all dropped-out cases were assumed to be poor responders, the good or excellent responders after 8 weeks of cisapride might be 48.0% of all recruited patients. The response to cisapride could not be predicted by the various clinical parameters of the patients. CONCLUSIONS Cisapride is an effective drug in at least half of the patients with chronic idiopathic constipation.