We performed a prospective, randomized trial in 52 patients to see how preparation with a single-dose sodium phosphate solution regimen (Fleet Phospho-Soda), in which the entire 89 ml dose was given on the evening prior to colonoscopy, compared with conventional polyethylene glycol(PEG)-electrolyte lavage in terms of quality of colon cleansing and ease of patient preparation. Previous trials had utilized a two-dose sodium phosphate regimen in which one dose was given on the evening prior to colonoscopy and the other on the morning of the procedure. A nurse gave a questionnaire to the patient prior to colonoscopy, assessing how well the preparation was tolerated. The attending gastroenterologist graded the quality of colon cleansing, unaware of how the patient was prepared or tolerated the preparation (1 = excellent, 2 = good, 3 = fair, 4 = unsatisfactory). Both preparations were similarly tolerated. The overall quality of bowel preparation with PEG lavage was significantly better (mean score 1.62) than with the single-dose sodium phosphate regimen (mean score 2.85) (P = 0.0002). Of 26 patients, 2 (7.7%) undergoing preparation with PEG lavage were judged to have had an unsatisfactory preparation, compared with 8 of 26 patients (30.8%) receiving the sodium phosphate regimen (P = 0.075). We conclude that the single-dose sodium phosphate regimen is suboptimal for clinical practice and its use cannot be recommended.