Collagenous colitis and microscopic colitis are histologic entities which do not have corresponding endoscopic features. Their precise incidence and role in the development of intestinal symptoms are poorly known. The aim of this study was to determine the frequency of these histologic abnormalities in patients with endoscopically normal colon and to correlate these findings with abdominal symptoms. Total colonoscopy was performed in 132 consecutive patients, 81 females and 51 males, aged 19 to 83 years (mean: 47.8 years). Patients complained of abdominal pain and/or diarrhea (66 cases), normal bowel transit or constipation (66 cases). Subjects were prepared for colonoscopy with polyethylene glycol 4,000. Three to 8 biopsies were taken from the rectum and the different parts of the colon. Histologic abnormalities were found in 36 patients (27.2 p. 100): collagenous colitis (7 cases), microscopic colitis (21 cases), and melanosis coli (8 cases). The frequency of diarrhea was significantly higher in patients with collagenous colitis and microscopic colitis than in those with melanosis coli or normal colonic mucosa. These results clearly demonstrate that routine biopsies of the rectum and colon are useful in patients with abdominal symptoms, particularly diarrhea, and normal endoscopy.