One hundred and sixty-two of 182 patients with coeliac disease provided satisfactory details of family size and the prevalence of coeliac disease and inflammatory bowel disease among their first-degree relatives. Patients ranged in age from 11 months to 79 years with a mean age of 41 (+/- 23) years. Twenty patients had at least one first-degree relative with coeliac disease: a total of 25 of 861 relatives were affected (prevalence = 2904/100,000) compared with an expected 0.9 cases (prevalence = 100/100,000; p less than 0.001). Six relatives had inflammatory bowel disease (prevalence = 697/100,000) compared with an expected 1.3 cases (prevalence = 150/100,000; p less than 0.001). Five of these had ulcerative colitis, and one had Crohn's disease. The relative risk of ulcerative colitis is, therefore, five times greater for first-degree relatives of people with coeliac disease than for the general population (95 per cent confidence interval, 4.7-7.2). There is a clear association between coeliac disease and ulcerative colitis, which may point to factors involved in the aetiology of colitis.