The relative risk of inflammatory bowel disease among parents and siblings of Crohn's disease patients.

Siblings of patients with Crohn's disease are 17-35 times more likely to develop the disease than members of the general population. Parents of patients with Crohn's disease are 35-70 times more likely to develop Crohn's disease than the general population. The crude risk of siblings or parents of patients with Crohn's disease developing ulcerative colitis may be half that of developing Crohn's disease; their relative risk may be a quarter that of developing Crohn's disease.