Fifty two first degree relatives of patients with coeliac disease were investigated for HLA status, small intestinal permeability, and mucosal morphology together with the size of the intraepithelial lymphocyte pool and indices of lymphocyte activation, in an attempt to identify genetically determined markers of the disease. Thirty eight per cent of these subjects had increased intraepithelial lymphocyte populations and a highly significant association with HLA-DR3 compared with controls. Their intestinal permeability to 51chromium-labelled ethylenediamine tetraacetate was invariably normal and there was no evidence of abnormal mucosal architecture, increased crypt cell mitotic activity or lymphocyte 'activation'. Although increased intraepithelial lymphocyte counts clearly do not cause alterations in intestinal structure or function, it is likely that this parameter together with the HLA-DR3 status identifies a genetically determined predisposition to the disease which may only become clinically evident with larger doses of ingested gluten.