AIM:To quantify the intraepithelial lymphocytes (IELs) and to document the membrane expression of CD4,CD8,TCRγδ and adhesion and/or activation-associated molecules (CD103,CD28,CD44,CD69,HLA-DR,CD95/Fas) in the duodenal mucosa of patients with functional dyspepsia (FD) in order to provide arguments for an immunological process in FD.METHODS:Twenty-six FD patients according to Rome Ⅱ criteria (20 were H pylori negative) were studied and compared to 12 healthy adults.IELs were isolated from five duodenal biopsy samples,then quantified by microscopy and flow cytometry while the membrane phenotypes were determined by cytofluorometry.RESULTS:Duodenal histological examination was normal.In H pylori negative patients,the number of IELs was not different from that in healthy controls.Median percentage expression of CD4,CD8,or TCRγδ and CD103,CD44,CD28,CD69 on CD3+ IELs,among the adhesion/activation associated molecules tested,was not different from that in healthy controls.In contrast,the median percentage expression of CD95/Fas [22(9-65) vs 45(19-88),P=0.03] and HLADR expressing CD3+ IELs [4(0-30) vs 13(4-42),P=0.041 was signifcantly lower in the H pylori negative FD group than in healthy controls,respectively.The number of IELs was significantly greater in H pylori positive FD patients than in healthy controls [median ratio for 100 enterocytes 27.5 (6.7-62.5) vs 10.8 (3-33.3), P = 0.02] due to a higher number of CD8+ CD3+ IELs.CONCLUSION: In H pylori negative FD patients, the phenotypic characterization of IELs suggests that we cannot exclude a role of IELs in FD.