In order to study the role of γ/δ T cells in the pathogenesis of inflammatory bowel disease (IBD) in humans, we measured the percentage of these cells in the peripheral blood, assessed the ratio of the non-disulphide-linked (δTCS1) type of T cell receptor (TCR) in the total γ/δ T cells, studied the co-expression of γ/δ TCR and accessory molecules CDS and CD 16, and compared these data with both the type and the activity of the disease. Percentage levels and absolute numbers of γ/δ⁺ T cells were higher in active patients than in controls (P < 0·05), mainly as a result of an increase of Vδ1⁺ (δTCS1) T cell subset (P < 0·05). This trend was strongly retained independently of disease activity and clinical picture. An increased percentage of TCRδ1⁺/CD16⁺ cells was observed in our patients compared with controls (P < 0·05). In contrast, no difference was observed as far as the TCRδ1⁺/CD8⁺ cells were concerned. These results suggest that IBD is associated with an expansion of γ/δ T cells in peripheral blood, which may play a role in the pathogenesis of these disorders.