Biopsy specimens from the terminal ileum of 32 patients with the histopathological diagnosis of lymphocytic colitis or collagenous colitis and 11 control individuals were evaluated for the presence or absence of ileal mucosal abnormalities and for the number of intraepithelial lymphocytes, assessed by immunohistochemical stains for the pan T-cell marker, CD3. We found that the mean CD3 counts in patients with lymphocytic/collagenous colitis were significantly higher than those in the control group. Seven of 14 patients with collagenous colitis and 14 of 18 patients with lymphocytic colitis revealed an increase in intraepithelial T lymphocytes when compared with the control group (P =.001). Other notable changes included ileal villous atrophy in one case of lymphocytic colitis and in three cases of collagenous colitis and epithelial damage with thickened subepithelial collagen in two cases of collagenous colitis.