Ulcerative colitis and Crohn's disease were studied in three Jewish populations in the Beer Sheva district of southern Israel. Age-adjusted prevalence rates on 31 December 1987 were for ulcerative colitis, 89/10(5), and for Crohn's disease, 30/10(5). Both diseases were commoner in females and in European-American-born Jews. Age-adjusted incidence rates have increased and in 1979-1987 were for ulcerative colitis, 5.4/10(5)/year, and for Crohn's disease, 2.1/10(5)/year. Females developed ulcerative colitis at a younger age and Crohn's disease at a later age than males. Israel-born patients developed both diseases at a young age. The clinical features of ulcerative colitis were similar in the various populations. Epidemiologic data in Jews may aid in the understanding of the pathogenesis of inflammatory bowel diseases.