One hundred and two patients were randomized to receive either oral sodium phosphate or standard polyethylene glycol-based lavage solution (Golytely) prior to colonoscopy in order to establish whether the much smaller volume of the former agent enhanced patient tolerance while maintaining or improving effectiveness and safety. Overall, patients found sodium phosphate preparation much easier to complete. In 37 patients who had had a previous colonoscopy prepared with Golytely, 100% of those now receiving sodium phosphate found it easier to complete and over 90% felt it caused less discomfort, compared with their previous experience with Golytely. Colonoscopists, unaware of the type of lavage solution used, scored the degree of colonic cleansing significantly higher for sodium phosphate-prepared colons compared with colons prepared with Golytely. Serial measurements of blood tests and postural pulse and blood pressure changes did not reveal any clinically significant changes in intravascular volume. Hyperphosphatemia was noted with sodium phosphate, but was transient, and no concomitant decrease in calcium was seen. Histological assessment for possible preparation-induced changes revealed no difference between the two agents. Sodium phosphate is significantly less expensive than Golytely. We conclude that, in the group of patients studied, sodium phosphate is a safe colonic cleansing agent that is better tolerated and more effective than Golytely.