We assessed the clonality of duodenal mucosal T cells in patients with celiac disease and controls. Fifteen adult patients were studied. Four patients had a complicated celiac disease, 3 did not respond to a gluten-free diet, and 2 had an ulcerative jejunitis (including 1 patient with nonresponsive celiac disease). Seven patients had an untreated celiac disease responsive to a gluten-free diet. Histological examination of duodenal biopsies of these 11 patients showed benign-appearing celiac disease without evidence of lymphoma. Four patients with nonulcer dyspepsia and normal duodenal biopsies served as controls. TCRgamma gene rearrangements were analyzed by multiplex polymerase chain reaction on DNA extracted from duodenal biopsies. Major clonal rearrangements of the T-cell receptor were found in 4 cases, all with complicated celiac disease. Monoclonality was confirmed by DNA sequencing of the junctional region in 3 cases and by hybridization with clone-specific oligoprobes. Patients with celiac disease responsive to gluten-free diet had mainly a polyclonal pattern, with 1 of them having an oligoclonal rearrangement. An oligoclonal pattern was also observed in 2 control patients. Three patients with complicated celiac disease evolved to T-cell lymphoma with liver (n = 2) or bone marrow (n = 1) invasion. Identical clones were found in the enteropathic duodenojejunum and peripheral blood in the patient with large-cell lymphoma with bone marrow invasion. This study suggests that complicated celiac disease is a cryptic T-cell lymphoma.