The authors' objective in this study was to determine geographic variations in the incidence of inflammatory bowel disease (IBD), specifically Crohn's disease and ulcerative colitis, in the Canadian province of Manitoba and its association with the sociodemographic, geographic, and disease-related characteristics of the study population. Using the University of Manitoba IBD Epidemiology Database, the authors applied spatial and ecologic techniques to visualize, explore, and model the incidence of Crohn's disease and ulcerative colitis for the period 1990-2001. The study demonstrated marked, statistically significant geographic variability in rates of both Crohn's disease and ulcerative colitis associated with the characteristics of the study population. Incidences of Crohn's disease and ulcerative colitis were observed to be highest among non-Aboriginal persons, persons of high socioeconomic status, persons with the lowest rates of enteric infection, and persons with the highest rates of multiple sclerosis. The observation of an inverse association between IBD incidence and rates of reportable enteric infection at the population level is consistent with the "hygiene hypothesis," which suggests that early exposure to enteric agents affords protection against eventual development of IBD. The positive association between IBD incidence rates and multiple sclerosis suggests that these two chronic, immunologically mediated diseases may have a common environmental etiology. This study underscores the importance of environment in IBD causation.