We prospectively studied the changes of colonic mucosa in patients receiving two different preparations for colonoscopic examination. Eighteen consecutive patients undergoing colonoscopy for polyps or mass lesions, properly age- and sex-matched, were randomized to receive Golytely lavage (3-4 L) or a standard preparation (48-h clear liquid diet, 240 ml of magnesium citrate and "X-Prep" senna derivative). Patients with diarrhea or inflammatory bowel disease, or both, were excluded. Biopsy specimens were obtained from normal-appearing mucosa of the right and left side of the colon (none in the distal 10 cm of rectum). Blind review of coded slides was performed with 0-3 scoring for artifact, edema and hemorrhage of the lamina propria, surface epithelial and goblet cells, crypts, and cells in the lamina propria including eosinophils and polymorphonuclear leukocytes. Statistically significant differences were found for preservation of surface epithelial and goblet cells and less edema in favor of patients receiving Golytely. We conclude that the standard form of colon preparation flattens the surface epithelial cells and depletes the goblet cells as well as causes an increase in lamina propria edema, whereas colon lavage preserves normal mucosal histology.