The efficacy of cisapride, a new prokinetic drug, as a treatment for chronic functional constipation of childhood was studied in 20 constipated children. Each subject had a stool frequency less than 4/week and/or total gastrointestinal transit time greater than 33 hr and was randomly assigned to double-blind treatment with either cisapride (N = 10) or placebo (N = 10) for 12 weeks. Stool habits, total gastrointestinal transit time, and anorectal motility were evaluated in all children before and at the end of the treatment period. Cisapride significantly increased stool frequency from 1.2 +/- 0.6 to 5.1 +/- 1.9 stools/week (mean +/- SD; P less than 0.05), whereas the lesser effect of placebo was not significant (1.2 +/- 0.8 to 2.8 +/- 0.8 stools/week; P = 0.4). Both treatments significantly (P less than 0.05) decreased laxative or suppository use. Total gastrointestinal transit time was decreased by cisapride (90.8 +/- 9.2 hr to 57.2 +/- 20.2 hr; P less than 0.05) but was not affected by placebo. Anorectal manometry showed that cisapride, but not placebo, significantly decreased the rectoanal inhibitory reflex threshold and the conscious rectal sensitivity threshold. It is concluded that cisapride improves gastrointestinal motility and bowel habits in children with chronic idiopathic constipation and may be useful in the management of some children with this disorder.