Two patients with "collagenous colitis" characterized by abdominal pain and chronic watery diarrhea are described. Colorectal biopsies showed a marked, band-like collagenous deposit under the surface epithelium. Transmission electron microscopy showed an abnormally thickened collagen table under a thickened basal lamina. Immunofluorescence showed no specific lesions. Radioimmunoassay of several gastrointestinal hormones revealed no abnormality. Examination of colorectal biopsies of a large control series (564 patients) did not show a comparable diffuse thickening of the collagen table in various types of inflammatory bowel diseases. Furthermore, no obvious age-related change of the collagen layer was found. The clinical history of our two patients is compared with three analogous cases, previously described. From our findings and those of others it can be concluded that collagenous colitis is a separate entity.