Methods: Postal questionnaire supplemented by review of case notes (for all patients). Interviews (personal or telephone) were carried out to confirm a positive family history.

Patients: Complete information was obtained from 433 adult patients with Crohn's disease. Ages ranged from 17-85 years. More than 95% of patients were resident locally.

Results: In 78 families (18%), at least one first- or second-degree relative also had IBD; three relatives were affected in 14 families and four in two families. Both Crohn's disease and ulcerative colitis occurred in these multiply-affected pedigrees. First-degree relatives were affected in 50 families (11.5%). Siblings were most affected (33 siblings in 29 families). Crohn's disease (20 siblings) was more common than ulcerative colitis (13 siblings); 19 parents and four offspring were affected. A spouse was affected in one family only. IBD was not present in families of adopted Crohn's disease patients. Assuming a population prevalence of 170/100 000 for IBD, the relative risks for IBD in siblings, parents and offspring of patients with Crohn's disease calculated from these data are, respectively, 24.7, 12.5 and 4.41.

Conclusion: These data confirm the increased prevalence of IBD in families of patients with Crohn's disease. Genetic susceptibility appears to be implicated in disease pathogenesis.