BACKGROUND: Oral sodium phosphate solution (NAP) and polyethylene glycol-electrolyte lavage (PEG-EL) are used for precolonoscopy bowel preparation. The benefit of adding cisapride to PEG-EL is controversial, and its influence on the effectiveness of NAP has not been investigated. The primary aim of this study was to determine whether cisapride improves the effectiveness and/or tolerableness of bowel preparation with either NAP or PEG-EL. METHODS: In 187 patients undergoing colonoscopy, a randomized, double-blind, placebo-controlled trial with a Latin square design was conducted to compare 4 different bowel preparations: NAP plus either cisapride (10 mg; 2 doses) or placebo, or PEG-EL plus either cisapride (10 mg; 1 dose) or placebo. Quality of the bowel preparation was graded by the endoscopist according to the amount of stool present in the colon (excellent, satisfactory, unsatisfactory). To assess tolerability, patients rated 8 symptoms, the taste of the lavage solution, and the ease of preparation on a 5-point scale (1: easy; 5: distressing). RESULTS: Endoscopists scored the bowel preparation as either excellent or satisfactory as follows: NAP: cisapride 50% versus placebo 61% (p = 0.3); PEG-EL: cisapride 80% versus placebo 78% (p = 1.0). Cisapride did not improve tolerability or the frequency of adverse symptoms associated with either solution. The ease of bowel preparation was significantly better in the NAP group versus PEG-EL group (mean score 1.8 versus 2.8; p < 0.0001). CONCLUSIONS: Cisapride does not improve the quality of bowel preparation with either NAP or PEG-EL. NAP is better tolerated by patients than PEG-EL; however, PEG-EL results in better bowel preparation.