Oral sodium phosphate (NaP) has been increasingly used for bowel preparation before the colonoscopy because it shows good patients tolerance and effective bowel cleansing ability. However, new studies describe that NaP can induce colonic mucosal damage. For better characterization of these changes, we examined histologically segmental colonic biopsies from 42 patients receiving NaP bowel solution before the colonoscopy. The series includes 25 male and 17 female patients in age from 19 to 81 years (average age 46.7 ys). Clinical symptoms in 37 patients included diarrhea, constipation, bleeding and abdominal cramps. The most frequent reason for colonoscopy was suspicion of microscopic colitis. Five patients underwent endoscopy to rule out the presence of neoplasia. None of the patients took drugs before the colonoscopy. Histologically, all specimens showed mild focal edema, hyperemia and hemorrhages. In addition to edema and hemorrhage, in 26 patients (61.9%), patchy mononuclear infiltration in the upper part of lamina propria and increased epithelial cell proliferation of individual crypts were seen. Mucosal structure was normal, with partial sloughing of normal or flattened surface epithelium. In 5 patients (11.9%), some biopsy samples contained scattered neutrophilic leucocytes in the lamina propria/superficial epithelium, isolated basal cryptitis, increased proliferation and apoptosis of the crypt epithelium. In two patients with focal cryptitis (4.8%), small erosions were found. Mild basal cryptitis, increased proliferation and striking apoptosis were present in two inflammatory pseudopolyps (in two patients). In 4 patients, solitary tubular adenomas with low-grade dysplasia without any reactive changes were found. In addition, 300 hyperplastic polyps removed endoscopically after the NaP application, were examined. Two polyps (0.75%) showed cryptitis and isolated multinucleated epithelial cells in the superficial part of the crypts. Our results are similar to those previously described in other studies of colonic changes after the NaP application. It reflects probably a similarity in composition of used NaP solutions.