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Background The Barwon area in Australia has one of the highest incidence rates of inflammatory bowel disease ( IBD) and therefore is an ideal location to study the impact of environmental exposures on the disease's development. Aim To study these exposures prior to the development of IBD in a population-based cohort. Method One hundred and thirty-two incident cases (81 Crohn disease ( CD) and 51 ulcerative colitis ( UC)) from an IBD registry and 104 controls replied to the International Organization of Inflammatory Bowel Diseases environmental questionnaire. This included 87 questions about pre-illness exposures that included childhood illnesses, vaccinations, breastfeeding, house amenities, pets and swimming, diet and smoking. Results The factors associated with CD included smoking (odds ratio ( OR): 1.42, confidence interval ( CI): 1-2.02, P = 0.029); childhood events, including tonsillectomy ( OR: 1.74, CI: 1.15-2.6, P = 0.003) and chicken pox infection ( OR: 3.89, CI: 1.61-9.4, P = 0.005) and pre-diagnosis intake of frequent fast food ( OR: 2.26, CI: 1.76-4.33, P = 0.003). In UC, the risk factors included smoking ( OR: 1.39, CI: 1.1-1.92, P = 0.026) and pre-diagnosis intake of frequent fast food ( OR: 2.91, CI: 1.54-5.58, P < 0.001), and high caffeine intake was protective ( OR: 0.51, 95% CI: 0.3-0.87, P = 0.002). Other protective exposures for UC included high fruit intake ( OR: 0.59, CI: 0.4-0.88, P = 0.003) and having pets as a child ( OR: 0.36, CI: 0.2-0.79, P = 0.001). Conclusion This first Australian population-based study of environmental risk factors confirms that smoking, childhood immunological events and dietary factors play a role in IBD development; while high caffeine intake and pet ownership offer a protective effect. [ABSTRACT FROM AUTHOR]