Thirty-four patients, aged 3 to 17 years, were randomized to receive oral sodium phosphate solution or a polyethylene glycol-based solution in preparation for elective colonoscopy. Nineteen patients received two doses of oral sodium phosphate solution (45 mL/1.7 m2/dose) and 15 received polyethylene glycol-based solution (4 L/1.7 m2). Compliance with oral sodium phosphate solution was judged as easy or tolerable in 15 of 19 patients, but only in 5 of 15 who were given polyethylene glycolbased solution. The quality of colon cleansing was rated by an endoscopist who was blinded to the colon preparation method used. The bowel preparation was excellent or good (only liquid remaining in the colonic lumen) in 18 of 19 patients who received oral sodium phosphate solution and in 6 of 15 who received polyethylene glycol-based solution. The incidence of vomiting was similar in both groups, but abdominal pain occurred more frequently in the polyethylene glycol-based solution group. Hyperphosphatemia developed in patients who received oral sodium phosphate solution (serum phosphorus = 2.3 ± 0.7 mmol/L (7.2 ± 2.2 mg/dL; mean ± SD), but only in 1 of 15 patients in the polyethylene glycol-based solution group. Patients did not exhibit symptoms of hyperphosphatemia and serum calcium concentrations were similar in both groups. In summary, oral sodium phosphate solution is better tolerated than polyethylene glycol-based solution for bowel preparation in children. However, hyperphosphatemia occurred frequently in patients who received oral sodium phosphate solution. Further studies are needed to determine the optimal dose for safety and efficacy for the use of these solutions in children.