We have reviewed the epidemiology of ulcerative colitis in Israel. Ulcerative colitis is twice as common as Crohn's disease, and has increased in frequency in both sexes and in all ethnic sections of the Jewish population since 1960. The disease is more prevalent in European- and American-born Jews than in Asian- and African-born Jews and Israeli-born Jews, but this difference appears to be decreasing in magnitude. With the exception of European- and American-born Jews, ulcerative colitis is less prevalent in Israel than in Jews living elsewhere. Major regional differences in disease severity by sex, and in the extent of colonic involvement, were detected. Colorectal cancer occurs less frequently in colitis patients in Israel than elsewhere. The peak age at onset is 15-30 years, with Israeli-born colitis patients having a significantly earlier mean age at onset. Genetic and environmental causative factors are incompletely understood. The disease has become more prevalent in the Arab population.