Sixty-two patients undergoing colorectal surgery or colonoscopy were prepared by three methods of whole bowel irrigation: nasogastric saline solution alone, nasogastric saline irrigation with oral mannitol, and oral mannitol solution without saline. The additional of mannitol to saline irrigation reduced the risk of sodium and water retention, which was eliminated by oral mannitol alone. The best mechanical preparation was achieved by adding mannitol to saline irrigation, but oral mannitol alone was judged more acceptable by the patients and less demanding by the nursing staff and was the preparation of choice for colonoscopy.