One hundred thirteen patients were randomized to receive either oral sodium phosphate (Fleet Phospho-Soda), lemon-flavored castor oil (Purge), or standard polyethylene glycol-based lavage solution (GoLYTELY) before elective colonoscopy. The study purpose was to confirm the efficacy of oral sodium phosphate and extend observations to include castor oil. Overall, patients reported that sodium phosphate and castor oil were easier to complete (p < 0.05). Scores for cleansing the entire colon as determined by endoscopists who were blinded to the cathartic agent were highest in patients receiving sodium phosphate (p < 0.02). Scores of left-colon cleansing for flexible sigmoidoscopy were equally high for the three methods. Scores for taste and symptom side effects were similar for each preparation. There were no recognized signs or symptoms of hypocalcemia in the sodium phosphate group. Because of the low cost of oral sodium phosphate combined with the lowest repeat endoscopy rate for inadequate cleansing, patient savings were projected to be $5000 per 100 patients at this center. Oral sodium phosphate is a cost-effective colonoscopy preparation that is better tolerated and more effective than the polyethylene glycol-electrolyte lavage solution or castor oil.