Prevalence of inflammatory bowel disease in family members of Jewish Crohn's disease patients in Israel. A familial study of 189 Jewish Crohn's disease patients was conducted in order to evaluate the risk for inflammatory bowel disease (IBD) in relatives of the patients and to try to understand better the genetic component in the etiology of the disease among Jews. One hundred fifty-seven patients filled out questionnaires that were verified by personal interviews. In 10 families (6.6%), a first-degree relative of the propositus was found to have IBD, seven Crohn's disease and three ulcerative colitis. Among first-degree relatives, siblings were more frequently affected: of 400, five had Crohn's disease and one ulcerative colitis. Among 304 parents, two had Crohn's disease and two ulcerative colitis, while none of the propositi's children had IBD. The prevalence of first-degree relatives with Crohn's disease was similar in the 98 and 45 families of Ashkenazi and non-Ashkenazi origin: 5.1% and 4.4%, respectively. The risk for siblings of the probands to be affected were also similar in the two groups: 1.5% and 1.8%; while parents of the probands were affected only in the group of Ashkenazi Jews. 