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Asthma and the inflammatory bowel diseases (IBD) each arise through complex interactions between genetic and environmental factors, and share many environmental risk factors. We examined the association between asthma and Crohn’s disease or ulcerative colitis.We performed a population-based case–control study using health administrative data from the province of Alberta, Canada. The odds of a diagnosis of asthma preceding the diagnosis of either Crohn’s disease (N = 3087) or ulcerative colitis (N = 2377) were compared with the odds of diagnosis of asthma among persons without IBD (N = 402,800) using logistic regression. Effect measure modification by age at diagnosis of IBD (16 years or less, 17–40 years, or older than 40 years) was tested using a likelihood ratio test.A diagnosis of asthma was associated with increased odds of incident Crohn’s disease (adjusted odds ratio [OR], 1.45; 95% confidence interval [CI], 1.31–1.60). No effect measure modification was observed for age at diagnosis for Crohn's disease (P= .42). However, we observed effect measure modification by age at diagnosis for ulcerative colitis (P= .0103), with an adjusted OR of 1.49 (95% CI, 1.08–2.07) among individuals diagnosed at an age of 16 years or less (OR) and an adjusted OR of 1.57 (95% CI, 1.31–1.89) among individuals diagnosed at an age older than 40 years. However, there was no association between asthma and ulcerative colitis among individuals diagnosed between ages 17 and 40 (adjusted OR, 1.05; 95% CI, 0.86–1.26).In a population-based case–control study, we associated asthma with Crohn’s disease, and with early and late-onset ulcerative colitis.