Besides a genetic predisposition, a causal role of various environmental factors have been taken into consideration in the etiology of inflammatory bowel disease (IBD). The most consistent association of environmental factors so far identified is the association between non smoking and ulcerative colitis (UC) as well a between smoking and Crohn's disease (CD). Other factors such as oral contraceptives, refined sugar, perinatal events, childhood infections, microbial agents, and domestic hygiene have been found to be associated with ulcerative colitis and Crohn's disease but further evaluation is required to confirm the consistency and to define the strength of the association. Recent data also suggest that measles virus may persist in intestinal tissue and early exposure to the virus may be a risk factor for development of CD. The further investigation of environmental factors on IBD and the explanation of their role is expected to open new avenues for basic scientific research and may lead to the development of a more rational approach to the prevention and treatment of IBD. The available data suggest that UC and CD are heterogeneous disorders of multifactorial etiology in which hereditary and environmental factors interact to produce the disease.