Asthma and the inflammatory bowel diseases (IBD) both result from complex gene-environment interactions and share genetic and environmental risk factors. Aims: To examine the association between asthma and the subsequent development of Crohn's disease (CD) or ulcerative colitis (UC). Methods: We performed a population-based case-control study using health administrative data from the province of Alberta. Incident cases of CD and UC were identified using a validated algorithm based on ICD9 (CD

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... 5; UC 556) and ICD10 codes (CD K50; UC K51). An 8-year washout period was used to distinguish incident from prevalent cases for individuals ≥10 years of age at IBD diagnosis; a 3-year washout period was used for those <10 years of age. Controls were derived from an age-stratified random sample of 10% of Albertans. The diagnosis of asthma was confirmed using a validated algorithm (ICD9 493; ICD10 J45). The odds of a asthma preceding the diagnosis of either CD (n=3,087) or UC (n=2,377) was compared to the odds of diagnosis of asthma among those without IBD (n=402,800) using logistic regression. Effect measure modification by age at diagnosis of IBD as defined by the Montreal Classification (≤16 years, 17-40 years, or >40 years) was evaluated using a likelihood ratio test. Age-stratified models are presented in the presence of effect measure modification. Logistic regression models were adjusted for age at IBD diagnosis (in the absence of effect measure modification), sex, rural/urban environment, and quintile of median income for individuals living in a dissemination area. A sensitivity analysis was conducted in which the diagnosis of asthma could either precede or follow the diagnosis of IBD. Results: A diagnosis of asthma was associated with increased odds of incident CD (adjusted OR 1.45, 95% CI 1.31 to 1.60) with no effect modification by age at diagnosis (p=0.42). Effect modification by age at diagnosis was observed for UC (p=0.01). Asthma was associated with ulcerative colitis diagnosed at ≤16 years of age (adjusted OR 1.49, 95% CI 1.08 to 2.07) and >40 years of age (adjusted OR 1.57, 95% CI 1.31 to 1.89). There was no association between asthma