The effects of prokinetic treatment with cisapride in patients with constipation-predominant irritable bowel syndrome (IBS) were evaluated in a randomized, double-blind, placebo-controlled study. Sixty-nine IBS patients were assigned to a 12-week treatment with either 5 mg cisapride or placebo t.i.d.; this dosage could be changed if necessary. The mean weekly number of days on which a stool was passed in the cisapride and placebo group increased to 5.3 and 4.4 (p less than 0.05) during weeks 8-12 of treatment, and the number of days with stools of normal consistency increased to 3.5 and 1.9 (p less than 0.05), respectively. At week 12, the reduction in severity and frequency scores for abdominal pain was significantly greater (p less than or equal to 0.05) in the cisapride group (60 and 61%) than in the placebo group (40 and 32%), as it was for abdominal distension (p less than 0.05). Cisapride tended to be better than placebo in diminishing flatulence. In 71% versus 39% of the patients the overall rating for the response to treatment was good or excellent at week 12. Cisapride was well tolerated. These results suggest that the drug will be useful for the management of constipation-predominant IBS.