Five normal volunteers, six normal first degree relatives of coeliac patients, and four patients with altered immunity (two primary biliary cirrhosis, one common variable immune deficiency, one immunoglobulin A deficiency) were studied both before and after a six week period during which their normal diet was supplemented by 40 g of gluten per day. Administration of the high gluten diet produced significant architectural changes in the jejunal mucosa of both the normal relatives of coeliac patients and the patients with altered immunity (p less than 0.01). In some cases changes amounted to severe partial villous atrophy. A significant increase in intraepithelial lymphocytes was also observed in the normal relatives of coeliac patients and in the normal volunteers (p less than 0.05); patients with altered immunity had high intraepithelial lymphocyte counts both before and after the high gluten diet. A significant decrease in xylose absorption occurred in the relatives of coeliac patients and in the normal volunteers (p less than 0.05). Two normal relatives and one patient with altered immunity had diarrhoea, which ceased on return to a normal diet. These findings indicate that excessive gluten intake can induce changes in the jejunal mucosal architecture in susceptible individuals who do not have overt coeliac disease.