OBJECTIVE: Recent evidence suggests the involvement of the upper gastrointestinal tract in ulcerative colitis (UC). By conducting a prospective controlled study, we explored the immunological abnormalities in the duodenal mucosa of UC patients. METHODS: Duodenal and colonic biopsies were collected from 24 corticosteroid-free UC patients and 21 controls. Colonization by Helicobacter pylori and positivity for anti-endomysial antibodies was an exclusion criteria. The severity of duodenal and colonic inflammation was determined by endoscopic and histologic scores. Morphometry was performed to measure the surface area to volume ratio (SV). Duodenal CD3(+) and CD8(+) intraepithelial lymphocytes (IELs) and lamina propria mononuclear cells (LPMCs) were detected by immunohistochemistry. RESULTS: Fifteen UC patients and 14 controls were Helicobacter pylori and anti-endomysial antibody negative and were thus included in the study. Microscopic duodenitis was reported in 4 of the 15 UC patients (26.6%), and in none of the controls. A significantly higher number of CD3(+) and CD8(+) IELs and LPMCs was found in UC patients than in controls. A significant positive correlation between the percentage of both CD3(+) and CD8(+) IELs and disease activity was found in UC patients. SV was significantly reduced in UC patients compared to controls, and inversely correlated with the percentage of CD8(+) IELs. CONCLUSIONS: The duodenum of UC patients is infiltrated by a higher number of CD8(+) IELs which correlates with the degree of villous flattening and disease activity, but not with extent of the colonic lesions. Further studies are needed to clarify whether the duodenum is a target organ in UC.