Good epidemiologic studies of Crohn's disease and ulcerative colitis are available from only a limited number of geographic locations and populations. A review of the data reveals some interesting facts. There is considerable geographic variation in the incidence of both diseases, which is more marked for Crohn's disease. When various populations living in one geographic location were compared, great differences became apparent with respect to both diseases. Studies performed in nine different populations during the last three decades showed a marked rise, up to 400%, in the incidence of Crohn's disease in each population. No such rise was documented for ulcerative colitis. Studies on Ashkenazic Jews in different parts of the world showed very marked variations in the incidence of both diseases, better documented for Crohn's disease. Both diseases are very rare in several nonindustrialized populations and their incidence rises with migration to an urban environment. All these data point to the preponderance of environmental factors in the causation of these two diseases of unknown etiology.