BACKGROUND/AIMS: This study compared the efficacy and patient's tolerance between those given a divided dose of a polyethylene glycol solution (PEG) and those given a stimulant laxative plus a reduced dose of PEG. METHODS: 190 consecutive patients for colon cleasing were randomized into 3 groups. In group A, 2 L of PEG was administered on the evening prior to the colonoscopy followed by 2 L of the same solution on the morning of colonoscopy. In group B, 2 L of PEG was administered in the morning only. In group C, 2 bisacodyl tablets (10 mg) were administered on the evening prior to colonoscopy and 2 L of PEG was administered in the morning. The patients completed a questionnaire to assess their tolerance to the bowel preparation before the colonoscopy. The endoscopists scored the adequacy of the bowel preparation using the Ottawa scale along with their satisfaction with the quality of the procedure. RESULTS: While 4 patients (6.7%) could not completely take the recommended dose in group A, all patients in groups B and C could take the recommended dose (p=0.012). The patients in Group B had a better tolerance and fewer side effects than those in Group A (p=0.01). A higher adequacy of bowel preparation was observed in group A than in group B (p=0.000) and there appeared to be a higher adequacy of bowel preparation in Group C than in Group B (p=0.06). CONCLUSIONS: The 2 L PEG solution only does not appear to be as effective as a bowel cleansing agent for colonoscopy compared with the divided 4 L PEG solution. No statistical difference in the side effects and efficacy was observed between the divided 4 L PEG solution and the combination of bisacodyl 10 mg with 2 L of a PEG solution.