OBJECTIVE The aim of this prospective, randomized, study performed in 60 outpatients was to compare 2 precolonoscopy bowel preparations: oral sodium phosphate (NaP) versus standard polyethylene glycol-based lavage solution (PEG). None of the patients met any of NaP exclusion criteria. All patients were prepared on the day prior to colonoscopy. A patient-questionnaire and measure of serum electrolytes (calcium, phosphate, sodium, potassium), pulse and blood pressure were used to assess tolerance and acceptability of the preparation. The quality of colon cleansing was judged by blinded endoscopists. RESULTS Patient's tolerance to NaP was superior to PEG: NaP preparation was easier to drink and feelings of abdominal plenitude occurred in a smaller proportion of patients. A potassium decrease, a sodium increase and hyperphosphatemia were observed in the NaP group but without clinical events. PEG preparation seemed to allow a better cleansing ability compared with NaP but this difference was not statistically significant. CONCLUSIONS NaP solution was better tolerated and accepted by patients. Colonic preparation quality compared to PEG is still to be discussed depending on the intake schedule. A biochemical data check seems necessary on account of significant serum electrolytes changes induced by NaP preparation.