The epidemiology of ulcerative colitis (UC) was studied in the Jewish population of central Israel in a densely populated urban area of more than 1,400,000 inhabitants. The mean annual incidence for the years 1970-80 was 3.86/100,000, 3.94 in males, and 3.79 in females. The incidence rose from 2.67 in 1970 to 5.09 in 1979, the rise being similar in both sexes. This rise in incidence was found in 3 separate localities in the study area inhabited by communities of different extraction and age composition. The disease started most frequently between the ages of 25 and 29. The crude prevalence of UC in 1980 was 55.2/100,000. The age-adjusted prevalence in Israel-born Jews was 45.8, in Asia-Africa-born 48.5 and in Europe-America-born 52.7/100,000. Compared with our previous study in 1960-70 in the city of Tel Aviv-Yafo, the prevalence of UC was increased and the differences between the community groups have narrowed. This suggests an effect of environmental factors in the causation of ulcerative colitis.