INTRODUCTION: Microscopic colitis (MC) is a chronic inflammatory process observed in colon biopsies of patients with chronic aqueous diarrhea. It is called microscopic because diagnosis is determined by histological studies since the microscopic characteristics of the colon endoscopy are normal. Two patterns exist: Lymphocytic Microscopic Colitis and Collagenous Microscopic Colitis. Etiology is unknown, and the proposed pathogenic mechanisms indicate an immunological phenomenon. Based on this, the authors of this study hypothesize that lymphocytic infiltration of the lamina propria could be related to cytotoxic lymphocytes CD8 as causative agents of colon tissue damage. OBJECTIVES: Prove hypothesis of immunological pathogenesis of MC. APPARATUS AND METHODS: Thirty eight (38) patients with diagnosed MC were recruited for the last four years in the Pathology Laboratory at Ricardo Palma University. Twenty two (22) colon biopsies with the most severe histological lesions were selected. These biopsies were obtained from 17 patients: 5 patients had 2 biopsies in 2 colonoscopy sessions. Biopsies were fixed in neutral formaldehyde, processed through the paraffin inclusion method, and stained with hematoxiline-eosine and Masson trichromic to distinguish collagenous tissue. Immunohistochemistry was conducted in 4- or 5-micron-thick histological sections processed through the immunoperoxidase method. RESULTS: Nineteen (19) biopsies corresponded to Lymphocytic MC and 3 to Collagenous MC. Lymphocytic MC showed intraepithelial lymphocytosis, dystrophic epithelial damage in the areas of lymphocytic infiltration, lamina propria inflammation with lymphocytes and plasma cells, and normal basement membrane. Collagenous MC showed thickened basement membrane due to the presence of a collagenous band, mild to moderate intraepithelial lymphocytosis, vacuolization,and frequent detachment of protective epithelium. Twenty two (22) biopsies were positive in the immunohistochemical studies.