An epidemiological study of ulcerative colitis was performed in the Upper Galilee, Israel, over a 20-yr period (1967-1986). The average annual incidence of ulcerative colitis was 2.23 per 100,000 population, and the prevalence on December 31, 1986, was 44.58 per 100,000. Considering the fact that strict steps were taken to include only definite cases, these figures are probably an underestimation. An increase of the average annual incidence from 0.88 in the period 1967-1976 to 3.79 in 1977-1986 was found. When the data were stratified according to ethnic groups, the highest average annual incidence and the highest point prevalence was found in Israeli-born Jews (6.9 and 138.2 per 100,000 population, respectively). When Jewish residence patterns were compared, the highest average annual incidence and point prevalence were found among Kibbutz members (5.52 and 110.39, respectively), and the lowest (1.94 and 38.76) among Moshav inhabitants. There were 10 Arab patients with an average annual incidence of 0.96 and a point prevalence of 19.27. There were 25 women and 28 men (female:male ratio of 0.89). Among the Jews, the female:male ratio was 1.04. Peak incidence was found in the 25- to 34-yr-old range. No second peak was noticed. Anemia was demonstrated in 66.6% of the women and 27.5% of the men in our study. We suggest that the increase in UC incidence and prevalence in Israeli and Asia/africa-born Jew and in Arabs in the Upper Galilee points toward environmental factors in the etiology of this disease.