Prevalence of ulcerative colitis in the Israeli kibbutz population. We carried out an epidemiological study of ulcerative colitis in 279 Israeli kibbutzim (121,403 population). The prevalence on December 31, 1987 was 121.08 per 100,000 population. When the data were stratified according to ethnic groups, the highest point prevalence was found in Israeli-born Jews (220.56 per 100,000 population), more than in Asian-African-born, or European-American-born kibbutz members (139.20 and 78.73 cases per 100,000 population, respectively). There were 68 men and 78 women (ratio of 0.87). The average age of the patients in the year of this survey was 46 years; it was 35 years at the time of diagnosis. Proctitis was found in 57%, left-sided colitis in 12%, and substantial or total colitis in 31%. Relapse at pregnancy was demonstrated in seven patients, and remission in one. Family history of a first-degree relative with inflammatory bowel disease was documented in three patients (2%). Probable complications of ulcerative colitis were observed in 38 (26%), anemia in 13 (9%). One patient (0.7%) with rectal cancer, also had breast cancer. We suggest that the impressive increase in ulcerative colitis prevalence among Israeli and Asian-African-born, in comparison with European-American-born kibbutz members, points toward a role of environmental factors in the etiology of this disease. 