In coeliac disease immunological abnormalities are not confined to the small bowel and it has been suggested that changes in peripheral blood lymphocytes may predispose to autoimmune or malignant complications. Using dual-colour immunofluorescence with labelled monoclonal antibodies, multiparameter flow cytometry was used to analyse peripheral blood lymphocytes in 32 untreated coeliacs, 29 treated coeliacs and 20 healthy volunteers. When the absolute numbers were considered, a decrease of CD3+, CD4+, CD8+ and CD19+ lymphocytes was found in untreated coeliacs compared with treated coeliacs and healthy volunteers. The proportion of CD3+ was significantly higher in untreated coeliacs (P<0.05) than in healthy volunteers. No differences were observed in CD4+, CD8+ and CD19+ subsets between the three groups studied. The proportion of CD3+ CD25+ and CD3+ HLA-DR+ cells were higher in untreated coeliacs (P<0. 001 and P>0.005) and in treated coeliacs (P<0.005 and P<0.05) than in healthy volunteers. On the contrary, natural killer cells and cytotoxic cells were lower in untreated and treated coeliacs than in healthy volunteers. As regards B-cell subsets, the only difference was the increase in FcepsilonR+ B cells in untreated coeliacs. The absolute reduction of peripheral lymphocytes in coeliac disease probably reflects their compartimentalization in intestinal mucosa. The decrease of natural killer cells and cytotoxic cells may be in keeping with the increased prevalence of malignancy in this condition. Finally, the phenotypic changes found in untreated coeliacs indicate T-cell activation.