It is increasingly recognized that chronic Ag exposure may lead to clonal expansions of T cells, including those within the peripheral blood. Inflammatory bowel disease is a chronic, multisystemic disease of unknown origin that predominantly affects the intestine. We sought to determine whether clonal expansions of T cells are present in the peripheral blood of patients with inflammatory bowel disease by an examination of TCR usage. Positively selected CD4+ and CD8+ peripheral blood T cells were isolated from subjects with active ulcerative colitis, Crohn's disease, and diverticulitis and from normal controls. Analysis of complementarity determining region 3 lengths of 24 TCR-β chain V region families from CD4+ and CD8+ peripheral blood T cells showed a skewed distribution in the three inflammatory groups, consistent with expansion of T cell clones, in comparison to the normally distributed pattern observed among the control donors. Random sequencing of the PCR amplification products of CD4+ peripheral blood T cells from the subjects with ulcerative colitis, Crohn's disease, and diverticulitis revealed reiterative TCR-β chain sequences that were not found in the normal donors. In subjects with Crohn's disease, the reiterative TCR-β chain sequences from the CD4+ peripheral blood T cells were persistent over at least a 1-yr period. The persistently expanded TCR-β chain sequences of CD4+ peripheral blood T cells were identifiable in genomic DNA isolated from archival tissue of intestine from subjects with Crohn's disease and ulcerative colitis by Southern blotting and direct DNA sequencing. An identical twin pair, concordant for Crohn's disease, shared the same reiterative TCR-β chain sequences in their CD4+ peripheral blood T cells. These studies show that chronic intestinal inflammation is associated with expansions of CD4+ peripheral blood T cells. Furthermore, in inflammatory bowel disease these T cell clonal expansions are persistent and shared among HLA-identical individuals, implicating a response to specific, persistent, and stimulating Ags in these diseases.