OBJECTIVES: The hygiene hypothesis postulates that individuals raised in a sanitary environment are more likely to develop inflammatory bowel disease (IBD). Several studies previously demonstrated contradictory results in this regard. We performed for the first time a population-based study on the association of surrogate markers of childhood hygiene with the risk for IBD.METHODS: A cross-sectional population-based study was undertaken. Information on number of siblings, birth order, and living in an urban versus rural environment was obtained for 399,251 Jewish adolescents at the age of 17 yr from 1998 to 2004. The study population included only subjects born in Israel. In order to control for genetic confounding, subjects were also divided according to their ethnic group as being Ashkenazi, Sephardic, or Israeli in origin.RESULTS: In total, 768 cases of inflammatory bowel disease (IBD) were diagnosed (0.19%), with 53.8% of the cases being of Ashkenazi origin. On multivariate analysis, variables significantly associated with IBD were male gender (odds ratio [OR] 1.44, 95% confidence interval [CI] 1.24-1.67), Ashkenazi origin (OR 1.91, 95% CI 1.63-2.22), living in an urban environment (OR 1.38, 95% CI 1.02-1.78), small number of siblings in the family (for 1 sibling vs 5 or more, OR 2.63, 95% CI 1.49-4.62), and higher birth order (for birth order of 5 or higher vs 1, OR 2.35, 95% CI 1.47-3.77), showing for both variables an almost strictly highly significant monotonic association (P value for trend <0.001).CONCLUSIONS: Surrogate markers of childhood hygiene are associated with the risk for IBD, thus reinforcing the hygiene hypothesis.