Background and study aims: Comparison of bowel preparation for colonoscopy in children with either Pico-Salax (sodium picosulphate with magnesium citrate) or polyethylene glycol with electrolyte solution (PEG-ELS). Patients and methods: In this investigator-blinded, randomized controlled trial, 83 children (12.5 ±3.1 years) requiring elective colonoscopy at a referral hospital were randomly allocated to Pico-Salax (n=43) or PEG-ELS (n=40), and an intention-to treat analysis was applied. Pico-Salax was administered in two doses, one the evening before and one on the morning of the procedure. PEG-ELS was administered over 4 hours. Efficacy was scored using the Ottawa scale and other constructs. Tolerability and toxicity were measured by patient and nursing questionnaires and serum biochemistry. Results: 35 of Pico-Salax patients (81%) were satisfied or very satisfied with the cleanout, compared with 19 (48%) in the PEG-ELS group (P=0.001). No differences were found in bowel cleanout effectiveness, as judged by the Ottawa score (P=0.24), completion rates (P=0.69), colonoscopy duration (P=0.59), need for enemas (P=0.25), or physicians global impression (P=0.7). Except for one case of mild dehydration in the Pico-Salax group, no clinically significant adverse events were recorded. Serum biochemistry results were similar between groups except for more hypermagnesemia associated with Pico-Salax and hypokalemia with PEG-ELS; neither was clinically significant. Conclusion: Children tolerate Pico-Salax better than PEG-ELS for bowel cleanout before colonoscopy. This study did not demonstrate superiority of effectiveness or safety for either regimen.