A double-blind cross-over trial of oral cisapride 10 mg before meals and placebo was performed to determine its effects on colonic transit in patients with severe idiopathic constipation. Nine patients with less than 3 spontaneous bowel movements/wk were studied. After passing a tube to the cecum, 50 muCi of 111In-DTPA were instilled into the cecum and followed for 48 h using colonic transit scintigraphy. In the group as a whole, cisapride had little effect on transit. The patients were then divided into two groups based on transit: functional rectosigmoid obstruction (FRSO) and colonic inertia (CI). In the CI group, cisapride accelerated the half emptying time of the cecum and ascending colon from 2.50 to 1.21 h (p less than 0.05). The progression of the geometric center was also faster after cisapride in CI. In FRSO, the geometric center was unchanged by cisapride except at 48 h. Cisapride thus has a prokinetic effect on colonic transit in patients with severe idiopathic constipation, colonic inertia subtype. It may be a useful agent in the treatment of this group of patients.