We performed a prospective randomized trial in a predominantly outpatient colonoscopy population to see how preparation with oral sodium phosphate solution compares with polyethylene glycol-electrolyte lavage in terms of the quality of colon cleansing, ease of preparation, and gastrointestinal intolerance. Before colonoscopy, a nurse administered a questionnaire to the patient to assess how well the preparation was tolerated (scale from 1 to 5:1 = easy, to 5 = unable to finish) and about the presence of four symptoms: abdominal pain, nausea, vomiting, and dizziness. The quality of colon cleansing was graded by the attending gastroenterologist, who was unaware of how the patient was prepared or tolerated the preparation (1 = excellent, 2 = good, 3 = fair, 4 = poor). The overall quality of bowel preparation with polyethylene glycol lavage was slightly better than with sodium phosphate (mean score, 1.93 vs 2.07); however, the difference was not statistically different. No statistical difference was seen in the frequency of patients with poor preparations (14.2% for sodium phosphate, 9.6% for polyethylene glycol lavage). Patients found preparation with sodium phosphate to be somewhat easier than polyethylene glycol lavage (mean score, 2.07 vs 2.41; p = 0.05). No difference was seen in the incidence of abdominal pain, nausea, or vomiting. Dizziness was more common with sodium phosphate but was mild and not believed to be clinically important. We conclude that the quality of colon cleansing is similar with polyethylene glycol lavage and oral sodium phosphate solution, with satisfactory preparation seen in 85% to 90% of patients. Patients found preparation with sodium phosphate to be slightly easier to tolerate.(ABSTRACT TRUNCATED AT 250 WORDS)