SummaryThe condensed article reports the results of 40 women and 16 men with a mean age of 59 ± 2.6 years diagnosed with collagenous colitis between 1980 and 1993 at the UCLA Medical Center. Diarrheal symptoms lasted from 3 months to 15 years and a total of 291 biopsies were obtained from four sites in the large intestine (rectum and sigmoid colon, n = 141; descending colon, n = 60; transverse colon, n = 46; cecum and ascending colon, n = 44). Fifty randomly selected specimens from 30 patients were independently investigated to test concordance of histopathologic interpretation. Thickness of collagen band, inflammatory cells, and epithelium were assessed by grading with H&E stained sections; measurement of the subepithelial collagen band on trichrome-stained sections was compared with the subjective grading. The results of the study appeal both to endoscopists and histologists: 1) If sigmoidoscopy is applied in the differential diagnosis of diarrhea, biopsies including the most proximal extent visualized, are mandatory. 2) Inflammatory changes parallel the collagen deposition. 3) Subjective sensivity in the estimation of collagen thickness in H&E stained slides is low. 4) Collagenous colitis without increase of inflammatory cells and a special type of collagen deposition is doubtful.