The effect of 10 mg oral bisacodyl (n = 19) or placebo (n = 26) on colon cleansing with polyethylene glycol electrolyte lavage solution was studied during colonoscopy. Patient tolerance was evaluated by questionnaire. During colonoscopy endoscopists graded adequacy of cleansing and subjective amounts of residual fluid and recorded the actual amount of fluid suctioned. Patient tolerance and adequacy of cleansing were not different between bisacodyl and placebo. There was no difference in subjective and measured residual fluid between the two groups. Compared to placebo, 10 mg oral bisacodyl does not influence adequacy of cleansing, patient tolerance, or residual fluid following polyethylene glycol electrolyte lavage solution for colonoscopy. This suggests that the beneficial cleansing effects seen with bisacodyl for barium enema may occur by mechanisms other than simple fluid evacuation.